Lulu is perhaps one of the most versatile champions in the game. Her kit and stats allow her to be played with a multitude of builds, with the most common ones being utility support and ability power carry.
The most common role for Lulu at the moment is of a utility carry mage. With a lot of AP items, Lulu becomes a force to be reckoned with, and while her damage is below the average damage for mages, her insane utility more than compensates for that. With Glitterlance, she can push the lane; with Whimsy, she can escape ganks. This allows her to push and harass her enemies, making them miss CS and allows her to become a lane bully, especially at the top lane, where she can harass most bruisers and tanks.
Don't make the mistake of understimating Lulu's damage just because she is primarily a support, because with good base damage, her passive, hard CC, a shield, and a Nasus/Renekton-esque ultimate, she can duel many champions very effectively.
Due to her enormous utility, Lulu can also be played as an utility support. She generally counters engage-based tanky supports such as Thresh and Leona because these supports have no sustain to keep up with Lulu's harass, and her abilities counter their engage. However, she fares poorly against sustain supports that are more passive, such as Soraka and Sona, although she still has an excellent late-game when compared to those supports.
When going support, pay attention to your team's needs. If your team lacks an AP champion for example, it may be better to buy a Spellthief's Edge, a Sightstone, then go full AP. If your team is in need of tanky champions, it may be better to build Lulu as a tank support. If neither is needed, however, Lulu can afford to make a build that balances utility with AP, so that she doesn't need to sacrifice either utility or damage.
There are a lot of builds that Lulu can use effectively aside from APC and support. She can be played with a Machine Gun build (a bruiser build that makes use of a lot of attack speed items to take advantage of her passive), as an ADC, and even as a full-tank.
It is important to note that, no matter the build Lulu decides to use, she never falls off late-game. Even if she loses her lane and is behind when compared to her enemy, Lulu has so much utility, that she will always be useful to her team.
Champion strength comes from her large amount of utility and high base damage from her skills. Pix, Faerie Companion encourages players to harass more with auto-attacks as it scales quite well, and her Glitterlance, Whimsy, Help, Pix!, and Wild Growth, have both offensive and defensive properties. Thus, decision making with her skills is vital to creating openings for her team to follow up on.
Glitterlance has a moderate mana cost at early ranks. Use it sparingly to poke, as it can quickly drain your mana.
You can use Help, Pix! on allies, enemies, or minions in order to greatly extend the range on Glitterlance and make an unexpected poke at an enemy champion.
However, for better mana efficiency, auto-attack a few times before casting Help, Pix! on an enemy. As they try to retaliate or escape, use Glitterlance on them to disengage or continue the attack.
Always be careful when using Whimsy. The speed boost on Whimsy is very useful, but using it this way may mean that it won't be available when you find that you need it to disable a high burst damage enemy such as Talon, or Riven, or to interrupt a channel such as Nether GraspMalzahar or DrainFiddlesticks.
Lulu excels at supporting initiation: attach Pix to a tank, then Whimsy them to allow them to move into range of the enemy quickly and fire off a Glitterlance to slow much of the enemy team.
use Wild Growth then and you will disable the entire enemy team for a couple of seconds.
A single rank Whimsy makes Lulu quite mobile, allowing for ganks or wards depending on her build.
Use Help, Pix! on enemies who can use stealth like Akali, Vayne or Twitch to keep them revealed and sabotage one of their biggest strengths.
Use Help, Pix! on high attack speed allied champions such as Master Yi, Fiora, or Evelynn as they will trigger your passive damage more often and since they are squishy most of time, they will benefit from the shield.
Wild Growth has many uses; it can enhance an already disruptive tank with its knockup and slow field, make a bruiser virtually impossible to peel off an enemy carry, or even protect a carry of your own. In lane, it can make ganks from champions with a leap or dash ability more potent, or to help escape enemy ganks. In general, it should be used early and often as its cooldown is far shorter than most ultimates.
If timed correctly, Wild Growth can be used to interrupt dashes or delay escapes long enough for your allies to finish the job.
Wild Growth does not grab turret or minion aggro when knocking up enemy champions as it does not do damage. Thus it can be safely used within turret range.
Wild Growth isn't affected by healing reduction because it increases health, not heals it.
In particular, Wild Growth synergizes exceptionally well with Yasuo, as it can knock up several enemies in a large area for him to use Last Breath on, along with a cooldown low enough to use it again in another fight.
Use Whimsy to help allies escape dangerous situations. Casting Whimsy on them or on the enemy will make for an easier getaway.
Since Lulu has high base damage but low ability power ratios on her offensive abilities, magic penetration and cooldown reduction benefit her a lot.
At 40% cooldown, you can constantly keep Help, Pix! on an ally.
Cooldown reduction also allows her to keep constant CC on enemies in teamfights.
As for magic penetration, getting an early Haunting Guise is a good, albeit not that common option on Lulu. The flat magic penetration allows Lulu to do a lot of damage to enemy champions early game, due to her base damage. You can upgrade it to Liandry's Torment, but it's generally recommended to do so only when you finish at least 2 other AP items.
Liandry's passive will deal damage for the duration of the slow from Glitterlance.
The health is also good as without one or two items that offer health, Lulu is very squishy.
Lulu is somewhat mana hungry. If you keep spamming her abilities early game without caution, she will become oom quickly. For this reason, it's recommended to buy an item that increases her mana regeneration or mana pool.
Most players opt to buy a Chalice of Harmony due to being a cheap item that instantly solves her mana problems, and the magic resistance is useful if Lulu is laning against an AP champion. When upgraded to Athene's Unholy Grail, this item also offers cooldown reduction, which is an stat that, as explained above, is essential for Lulu.
An alternative to the above item is Rod of Ages. This item lacks mana regeneration, but the extra mana pool compensates for that. However, this item takes a lot more to reach its full benefits, when compared to Athene's Unholy Grail. It also lacks the cooldown reduction. The main reason why Rod of Ages is a good option, however, is because it makes Lulu a lot tankier when compared to Athene's. This item is actually recommended over Athene's if Lulu is laning against an AD champion.
Another option is Morellonomicon. This item offers much less mana regeneration than Athene's Unholy Grail, and doesn't have any defensive stats. However, it offers slighty more AP, and the passive is great against sustain-based champions, such as Swain and Yorick.
Since Lulu is somewhat auto-attack based, and she only has 2 offensive spells, she makes great use of a Spellblade item.
Lich Bane adds to Lulu's damage, making her surprisingly bursty, and she can very often proc this item's passive due to her low cooldowns, especially if she builds cooldown reduction. The extra movement speed also helps, as Lulu has a below-average base movement speed.
Iceborn Gauntlet is also a viable alternative, due to the armor this item offers, making Lulu surprisingly durable against AD opponents. It also has other good stats, such as cooldown reduction and mana. The Spellblade this item offers is different; while it does much less damage when compared to Lich Bane, it slows down the enemy, making easier for Lulu to land her Glitterlance, and also adds to her chasing and peeling power.
Keep in mind however, that Glitterlance already slows her opponent. In addition, Lulu has one of the lowest base attack damage on the game, and since this item does not scale with Lulu's ability power, the damage it deals is very weak. While Iceborn Gauntlet has AP, it is not sufficient to boost Lulu's overall damage. If you are purchasing Iceborn Gauntlet, you should be purchasing it for the extra armor and cooldown reduction.
Nashor's Tooth is also a good item to buy, and an alternative to Lich Bane, especially if you decide to buy Rod of Ages, to compensate for not having the cooldown reduction that Morellonomicon and Athene's Unholy Grail offers.
This item combos well with Lulu's passive, adding even more on-hit magic damage, and increases her overall DPS, due to the extra attack speed.
Lulu scales well with attack speed, due to Pix, Faerie Companion. If Lulu purchases on-hit attack speed items, not only she can trigger her passive more often, but her DPS will increase dramatically.
Keep in mind that Runaan's Hurricane will NOT allow Pix, Faerie Companion to strike multiple enemies with the secondary bolts, he will only attack Lulu's primary target.
Frozen Mallet is also a good option for such build. Despite offering no attack speed, this item offers health, and an on-hit effect that slows down opponents with her basic attacks, allowing Lulu to bombard her opponent multiple times with Pix, Faerie Companion.
This build is known as "Machine Gun Lulu", and is usually played at top-lane. This build is considered sub-optimal by some players, because AP items typically don't have synergy with most of the aforementioned options. However, "Machine Gun Lulu" has two advantages over a normal AP build. The first is that it allows her to beat some of Lulu's counters, such as Vladimir. The second is that this build allows Lulu to purchase some tanky items, making her somewhat of a bruiser.
Although Lulu has low AP ratios, Rabadon's Deathcap will still significantly increase her damage, as well as making her utility spells stronger.
As an AP support, Lulu can pick up Spellthief's Edge as a good starting item since her kit encourages her to auto-attack and harass. The active of its final upgrade, Frost Queen's Claim, can allow you to catch out enemies from a longer distance.
Lulu can easily support other champions from the back, making aura items a viable choice.
Mercury's Treads reduces both the duration of her disables, as well as her damage output, making it an ideal item.
Lulu does not have a very high natural damage output, so when alone or focused, she'll die quickly without much retaliation.
However, Lulu has a much higher damage output than most other support champions along with her terrifying amount of CC. Don't ignore her in team fights.
Despite Champion strong crowd control, poke, and peeling power, she does not have any form of heals to keep the AD carry healthy except for her ultimate, and her abilities are mana intensive. Therefore, support champions with healing like Sona, Soraka, and Nami can stay in lane longer.
Champion true power comes from the CC she can apply, and as such, is extremely dependent on her teammates; her damage doesn't scale well into the late game, due to her medium ratios, but remains a powerful disabler.
By late game Whimsy can give a very high speed boost depending on her build, and can be used quite often in conjunction with CDR. Beware if she uses it on a power initiator as it could mean an easy engagement.
In lane, Lulu will often harass heavily with her Glitterlance and her passive. Unlike many piercing skillshots, standing behind minions will not mitigate the damage, so moving erratically can help mitigate damage taken.
Lulu has no consistent heal, so poking at her or her lane partner will whittle down their health or waste her mana trying to shield them.
Lulu has mediocre auto-attack damage (44.4) as her passive gives her more damage on attacks and it quickly scales as she levels up. Do not be afraid to exchange auto-attacks early with Lulu as champions often have more magic resist than armor early on. (Pix will barely deal about 5.5 damage at level one)
Be wary that Pix will become a threat if Lulu has built a lot of attack speed. The damage scales quickly, and getting hit by all three bolts lets her outdamage carries during the laning phase. By level 17 Pix adds up to 105 magic damage per auto (not accounting for AP), and becomes even deadlier when used on a carry with higher attack speed. By this time champions don't have much magic resist unless they have itemized for it, making it deceptively powerful.
Wait for Lulu to use Whimsy before trying to use any channeled abilities or heading into the fight. This leaves her without the polymorph or speed buff, and allows your team to fight without disruption. It has a fairly high cooldown, especially at earlier ranks, meaning you have a good amount of time to fight before Lulu can use it again.
When she and her lane partner hit level 6 it can be very hard to beat them in a 2v2 situation as Wild Growth delivers a devastating disable and heals her target for a significant amount.
Like many ultimates on supports, Wild Growth has a somewhat high mana cost and can be rendered unusable if you manage to harass her into draining her mana pool.
Lulu is an excellent baiter due to Wild Growth; be very careful about diving or committing to kill her if she has the ability up.
Watch out for Help, Pix! as Glitterlance's second bolt will fire from Pix's position, meaning that Lulu can pick up an unexpected kill on you or a nearby ally if she tags you or a nearby champion/minion with it.
Despite Lulu being squishy, she is extremely hard to catch, being able to escape with Whimsy, slow you with Glitterlance, prevent some damage with Help, Pix! and if you get too close she will send you flying with Wild Growth while the slowing field will keep you from catching up.
Try to get Lulu to waste her ultimate without her team initiating. If Lulu uses Wild Growth in conjunction with a great initiating champion such as Malphite or Maokai, your team will be locked up for a few seconds.
If she does use it, be careful of what you do next.
While the knock up, health, and slow are disruptive, they don't hurt your team much after the initial knock up, so staying and fighting may be a good option if you can take out the champion before Lulu can shield them or give them the ability power and speed buff from Whimsy.
If you have to run, beware of the slows from Wild Growth and Glitterlance. If you get slowed by both of them, a polymorph from Whimsy may take you out if Lulu's team is nearby. Also keep in mind that Help, Pix! extends Glitterlance's range, making it a very effective chasing tool if she attaches it to you or a nearby champion/minion.
Finally, remember that Help, Pix! reveals a tagged enemy champion until Pix leaves, making it impossible to juke through bushes or past corners for the duration of the debuff.
Lulu is one of the few champions that has strong flexibility in her item builds and kit in a similar vein to Kennen and Teemo. Pay attention to her build and build accordingly to counter her. For example, if she goes for a "machine gun" type build where she builds a lot of attack speed and on-hit items with the use of Pix, Faerie Companion, consider building Randuin's Omen and Frozen Heart to counter her.
On the other, pay attention to which lane Lulu can go. While she's normally played as a support, she can also be played as an ADC due to her Pix, Faerie Companion and disabling kit. She can easily go top and middle lane due to her crowd control and poking kit in addition to Wild Growth and Whimsy's speed boost makes it incredibly difficult for melee bruisers and assassins to kill her quickly. Consider playing sustain based champions like Nidalee, Yorick, and Vladimir instead to counteract her crowd control and poke.