Carnivore allows Cho'Gath to maintain his health and mana, making him a great solo laner. Note, however, that the mana restored is fairly low - it takes about 4 full minion waves to pay for 1 cast of Rupture.
Carnivore can save you from Ignite if there are enemy minions at low health nearby, even with the healing reduction effect. If you have enough AP, ignited and probably nothing to lose, you might try killing Lesser Wraiths with Rupture.
Rupture's powerful disruption and the fact that none of Cho'Gath's skills have reduced cooldowns as they are leveled makes cooldown reduction very valuable. Place Rupture in the path of any moving enemies, as it is a delayed ability.
Its delayed nature and the hard CC can be used to your advantage as it can interrupt otherwise deadly suppression ultimates like Infinite Duress and Nether Grasp, provided that you can anticipate when they are going to be used. This is especially powerful against Infinite Duress as it is a common gank initiator and is more predictable, so you can interrupt it by casting Rupture under yourself.
Remember however, that when saving a teammate instead of yourself, Feral Scream is faster and more efficient.
Because of Rupture, Cho'Gath is at his most powerful if he can bait the enemy into chasing in a disadvantageous situation or intimidate them into fleeing.
Feral Scream is a strong AoE damage/silence ability, especially in conjunction with Rupture. Due to his basic attacks being melee, you can first use Rupture to knock up your target, then close the gap sufficiently within the time to silence them with Feral Scream, preventing effective escaping or retaliation. Due to the strong slow applied by Rupture, it is unlikely they will be out of melee range before the silence expires, allowing you to finish them off with Feast if possible.
Note that if you attempt to fire Feral Scream in the opposite direction of your movement, it would often be launched in the wrong angle and become counterproductive.
Getting at least one rank of Vorpal Spikes is a good way to farm minions. Because of the linear-splash damage with each auto-attack, this is good for triggering Carnivore.
It can sometimes be a good idea to toggle off Vorpal Spikes to prevent yourself from pushing the lane too far and thereby opening yourself up to ganks. If you don't toggle it off when attacking a turret, an enemy champion can stand near the tower where you will hit them due to Vorpal Spikes'sarea of effect damage, causing the turret to attack you.
If you're having difficulty Feasting upon champions, try eating minions until you're larger, as the true damage you deal exceeds the health of most minions. It is advisable to gain 6 stacks first by Feasting on minions and using it to last hit enemy champions that are too close, since the damage to minions is significantly larger (particularly in early-mid game, since Feast has a base damage of 1000 against minions, regardless of Feast's level).
This allows Cho'Gath to kill the Ancient Golem very quickly once he hits level 6. If he manages to secure the blue buff, he would be practically immovable from the lane due to it allowing him to constantly spam Rupture.
If you want to push your lane faster, it's better to Feast a Siege Minion for an instant minion kill since they're harder for your smaller minions to kill. However, note that as minions gain health over time, if you have 0 ability power, their maximum health would become higher than 1000 after 12 minutes into the game, preventing you from gaining a Feast stack.
If 20pxCho'Gath gets Feast stacks before level 11 or 16, he will be healed for the difference in the bonus health when he levels Feast.
A red particle effect will appear underneath champions that will die from Feast.
Building 20pxCho'Gath as an AP burst mage is reasonably viable because of his high AP scaling, and a true damage ultimate that could easy hit over 1200 on any target with sufficient AP. His main weakness is he must go into melee range to unload Vorpal Spikes, proc Lich Bane and use Feast. However, with the 100% AP scaling on Rupture, it would be better for him to use it as often as possible, get a Feral Scream once in a while, and execute weakened enemies with the aforementioned damage sources.
AP 20pxCho'Gath has one of the most formidable AoE bursts in the game along with the strong cc applied. However, he is extremely reliant on landing Rupture, and Feast is best used to turn around close-quarter fights when you can't keep your opponent's at arm's reach with Rupture.
Because Feast does a large amount of true damage, it is good for killing tanks that naturally stacks magic resist, like Galio. Other good targets include champions who are strongest when they are at low health like Olaf or Tryndamere, who can dish out tons of damage and heal themselves for extended periods of time otherwise. It's true damage goes through the Spirit Visage the former often builds, and the red circle indicator helps killing Tryndamere immediately before he can activate Undying Rage.
The true damage and health gain of Feast along with the continuous damage of Vorpal Spikes allows AP Cho'Gath to fare reasonably well against tanks in general.
20pxCho'Gath can be an extremely effective pusher. Use Rupture to kill the 3 melee minions, and Feral Scream to kill the 3 caster minions, then kill the siege minion with Feast if necessary. Vorpal Spikes helps by finishing off multiple minions with small slivers of health remaining. Be wary however, as he is very vulnerable to ambushes deep into enemy territory.
Rupture has a fairly long range and reveals a large area around it. Use it to check for enemies, but be wary about the high mana cost early game.
20pxCho'Gath synergizes well with a CC-heavy team; while he has respectable CC, landing them without any help or the element of surprise is quite often difficult. If his team can lock down the enemy team for just a short while, or even draw them in with the likes of Apprehend, 20pxCho'Gath could lay down even more CC much easier and inflict heavy AoE damage in an AP build.
If your team has a tank already, 20pxCho'Gath can switch to an off tank ability power build in order to do more damage. Since Feast already increases 20pxChampion maximum health, he will still be difficult to take down.
However, Rupture's range is noticeably longer than the Scepter.
Since Feast20pxCho'Gath deals true damage to champions as well as minions, a caster build on 20pxCho'Gath can allow him to deal extreme damage to single targets and can easily execute unaware enemies especially late-game with his high amounts of AP.
Investing in cooldown reduction items such as Glacial Shroud (or Fiendish Codex in an AP build) early allows more frequent use of his ultimate ability to build Feast stacks quickly and/or to threaten enemy champions with its high burst damage more often.
Rylai's Crystal Scepter is not as effective as it may be, as all his basic abilities deal area of effect damage, and his longest-ranged ability, Rupture, already includes a knock-up and a large slow. However, as Vorpal Spikes is added to all his basic attacks, it could force his opponents to use their movement abilities to get out of his melee range only to be caught up by Rupture.
Although Cho'Gath's cooldowns are somewhat high, he makes very good use of Iceborn Gauntlet due to having the highest base attack damage on the entire game. The slow will allow him to stick to this enemies easier, and Cho'Gath also makes good use of every stat Iceborn Gauntlet offers.
Keep in mind that toggling Vorpal Spikes will NOT activate Spellblade.
Lich Bane is arguably a better option than Iceborn Gauntlet on builds that focus more an AP than tankiness. Keep in mind that Lich Bane lacks both the slow and the armor from Iceborn Gauntlet; but it will do a lot more damage and will benefit from Cho'Gath's magic penetration.
Although Feast deals true damage, Cho'Gath still benefits from building magic penetration, since his basic spells still do magic damage. Thus, items like Sorcerer's Shoes and Void Staff are good options for him.
Attack speed items are a good idea, because they allow Cho'Gath to activate Vorpal Spikes a lot more times.
Wit's End is the best option for a tanky build, since it adds even more on-hit magic damage, and by reducing the enemy's magic resistance, Vorpal Spikes (as well as Cho'Gath's other abilities except Feast) will do more damage.
If you are using an AP-based build, Nashor's Tooth may be a better option.
However, relying on Vorpal Spikes for damage may not be the best idea, since it forces Cho'Gath to always be in melee range, and he has difficulties sticking to is enemies, especially if he can't consistently land Rupture. This can be solved by building a Rylai's Crystal Scepter.
Try to zone 20pxCho'Gath when he is low on health/mana from getting last hits, to prevent effective use of Carnivore. It may force him to push the lane because all his ranged attacks deal area of effect damage. However, if left alone he would quickly heal up. This, combined with his low mobility, encourages you to call for a gank and commit to killing him instead of harassment. Turret-diving him may be ill-advised, as the powerful CC from Rupture will prove deadly especially in early game.
Try not to stand close to minions at low health as it would allow him to simultaneously last hit, heal and harass you.
If 20pxCho'Gath is going AP, try to shut him down in early-mid game, and gank as much as possible. He is very strong at mid-game farming with a few AP items, and the assists granted from three of his AoE spells along with his ultimate that marks targets to execute, make it relatively easy to catch up even if he has a bad early game.
Remember to keep a distance from each other especially when ganking 20pxCho'Gath, otherwise it is asking for him to AoE CC and turn the gank on you.
AP 20pxCho'Gath is one of the very few champions in mid game that could push faster than minions can move. Ganking him often would make him wary of ambushes and relive a lot of your lane pressure in mid-late game. Try to ambush him right after he uses Rupture for maximum chance of success.
Even super minions would have a hard time slowing him down, as his ultimate does increased damage against minions and has a short cooldown. Unlike most mages, he also has strong passive sustain when pushing thanks to Carnivore.
Rupture is Cho'Gath's strongest non-ultimate ability, and it is critical to avoid it by watching for its animation. Before the spikes and the smoke appear, Cho'Gath stomps his foot on the ground. While 20pxCho'Gath lacks any conventional gap closers and escapes, Rupture's knock up and slow is more than enough for him to catch up or run away.
There is also a red/purple circle around Rupture making it easier to spot and avoid.
Due to his poor mobility, Cho'Gath is very vulnerable to kiting as long as you avoid Rupture, which also has a fairly punishing mana cost. However, during mid/late game team fights, avoiding it becomes much harder, making Cho'Gath a very dangerous AoE threat if you cluster around his team's initiator.
Try to move towards him when he uses Rupture, as he tends to target slightly behind you, and barely escaping a Rupture can lure him into a bad position.
Note however, Bouncing Bomb doesn't always go through minions; Wild Cards may not do enough damage to counter his passive, and is fairly easy to dodge.
All of Lux's abilities out-ranges it and Prismatic Barrier allows her to shrug off most of his harass; just be careful not to be silenced.
Due to its delay, mobile champions like Riven and Akali would also help against him.
Master Yi in particular, as his Highlander allows him to ignore Rupture's slow, given that it could even hit him with his incredible speed. As Cho'Gath grows, it is increasingly difficult to correctly aim his abilities at small-sized melee champions that can whittle him down. Due to his lack of long-ranged targeted abilities, un-targetable abilities like Alpha Strike or Blade Waltz generally give him fits.
Fizz has both un-targetable effects and the mobility with Playful.
Gragas could match him in sustain, his Barrel Roll will zone him better than he could do in return, and also has the mobility with Body Slam.
Save your channeled spells until he uses his two CC spells. However, at max cooldown reduction Rupture's cooldown is only 5.4 seconds, and a Cho'Gath alternating between the two would be very difficult to channel against.
Always try to bait Rupture before going for the offensive; as Feral Scream is a conic skillshot, abilities that transport you behind him such as Shunpo or Cutthroat can be very effective. Coincidentally, both of the mentioned abilities have a range of 700, same as Feral Scream, meaning that he shouldn't be able to prevent you from using it with his silence.
Be wary of chasing a weakened champion into brush when Cho'Gath is around. It will make it much easier for him to land Rupture and the rest of his combo on your entire team, as well as negating his lack of a gap closer to kill you with Feast.
Build magic resistance to mitigate his damage, but know that Feast deals true damage, not magic damage, and building health helps against true damage.
Also note that practically all of his abilities deal magic damage, and that his autoattack are most greatly augmented from the bonus magic damage that Vorpal Spikes provides. Items such as Spirit Visage or Warmog's Armor can greatly negate his ability to fight you one-on-one. Banshee's Veil will also make it difficult for him to initiate on you.
Two common anti-mage AD items both have reduced effectiveness against AP Cho'Gath. The active on Mercurial Scimitar has no effect on Rupture's knockup, the shield on Maw of Malmortius has no effect against the true damage from Feast, and it also ignores the magic resist granted by both items.