Evelynn is an effective roamer. Use Shadow Walk to scout out and surprise enemy champions that overextend and possibly get a kill. This is most effective early game. By roaming, this allows your solo laner to farm minions while keeping the enemy solo back and losing them gold and experience.
Evelynn can excel at pushing a lane and jungling by spamming Hate Spike. Try to stand so it hits many creeps.
Level up Ravage first for an AD/On-hit build and Hate Spike first for an AP build.
Use Ravage on a nearby minion when attacking a tower.
Your Shadow Walk can be easily neutralized by Vision Wards early game. Don't be too hasty in attacking early. Take your time, scope out the battle before joining and use your element of surprise to your advantage. Purchasing a Scrying OrbScrying Orb will enable you to see Vision Wards in bushes before you are even noticed.
Use Dark Frenzy to return to and move among lanes faster. Be careful using this before a gank in case you need it to escape.
With a maxed out Ravage attack speed boost, you need a 118.8% boost to reach the cap.
Evelynn is a great scout as she can walk past regular Sight Wards without being seen. Be wary of Vision Wards, as their true sight can detect stealth units, however they can be seen so you can find them, destroy them and come back shortly for a gank.
Since Agony's Embrace is based on the current health of the enemy and has no minimum damage, avoid using it on low-health opponents. The damage will be incredibly low.
For that reason, if you are going to use Agony's Embrace, always initiate with it, so you do the most damage, before your opponent's health is down.
Guinsoo's Rageblade because of your ability to quickly gain stacks making it very cost effective for a DPS build, but not a burst build since you won't have the stacks when you need to burst someone.
With both AD and AP, her AS buff can effectively mix and match any offensive item. See what defenses the enemy is building so you can adapt within your build. For instance, if they build Thornmail when you're playing Hybrid, build Wit's End instead of Trinity Force. If they all build magic resist then avoid building AP. If they build health, then Deathfire Grasp with AP + CDR for your ult can help, but items with sustain such as Hextech Gunblade or Blade of the Ruined King can let you fight tanks indefinitely, and can also be a good choice. Some good items are:
For mixed melee and spell damage, single and AOE targets with a mix of survivability as well as mid-high armor, magic resistance and hit point targets you can utilize an item set such as: Iceborn Gauntlet, Liandry's Torment, Spirit of the Elder Lizard, and The Black Cleaver. This will maximize damage penetration as well as provide defense, crowd control and destroy high hit point targets with high efficiency as well as allowing her to wave clear minions and monster camps with extreme ease. It will also apply all three types of damage while providing two DoT's to an escaping target. Finishing the build off with Rylai's Crystal Scepter and a pair of Sorcerer's Shoes provides maximum damage and maintains the target/target's will never escape while producing more damage AND survivability.
Note that Evelynn is inherently squishy, so any melee oriented builds must have some items devoted to tankiness, such as Iceborn Gauntlet or Randuin's Omen.
Actually, all builds require Evelynn to get very close to blow her target, particularly if she wants to hit Ravage that is her main instant nuke by late game, both on AP (very high ratio) and AD (high ratio) she has to nearly get to melee range. If you notice focus into you, for AD the aforementioned items might suffice, but a Frozen Mallet makes Evelynn more undesirable to focus and resilient to upfront burst, it offer little damage however and should only be obtained if under heavy focus
Since Ravage gives you 120% attack speed bonus, you can reach the 2.500 attack speed cap very easily. Try to not get more than about an additional 100% AS.
Speed items, such as Phantom Dancer andBoots of Mobility allow you to gank a lot faster and gain extra kills, keep enemies more uncertain where you are, and help you escape if you're seen out of position. Keep in mind that slows are more powerful than Hastes (Movespeed buffs), and that movespeed items do not give much tankiness, so if you are caught or like going into melee range, a tankier item would be better thanks to the movespeed bonus from Dark Frenzy.
Think twice before buying enemy affecting aura items such as Frozen Heart as the enemy can notice these effects from a distance, alerting them to your presence and possibly ruining your surprise. Ally affecting aura items can also be noticed, but with more difficulty. This is not an issue with Abyssal Scepter due to this item having only 600 range, while Evelynn's stealth range is 700.
Always look out for Evelynn in your lane. Because of her stealth, she can easily sneak behind unsuspecting players who are overextended. Be aware that her movement speed boost can make her very surprising.
You can protect your lane by purchasing Vision Wards and placing them in the river. Be careful if using Sight Wards as if she comes into your brush while already stealthed, you will not be able to see her until she unstealths, and that may be too late.
Evelynn can be built with either Ability Power, Attack Damage or even a mix of both. Pay attention to the way she builds to better decide what defensive items you should buy. However, she still deals a lot of magic damage even with an Attack Damage build, so Magic Resistance is rarely a bad option against her.
If Evelynn is building AD, she has a very weak early and mediocre mid, but trades for extremely high damage late game. If you are a squishy champ, stay behind tanks and try to focus her down.
Evelynn's stealth is lost when she takes damage. Harassing her greatly reduces her sneaking ability.
If you think Evelynn is near, use skillshots or AoE spells (Like a Bouncing BombZiggs or Mystic ShotEzreal) in her direction. If they hit it will disable her passive and reveal her.