Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Cho'Gath that doesn't straight tank and uses AP is really a powerful champion if used properly, however everyone sees him as a tank. He is dynamic in that he can become powerful with the correct skill/spells/mastery/runes/items as a tank or caster using ability power.
Using this build, and being skilled with:
-Feasting as a kill shot and getting stacks up quickly
-Timing screams to prevent opponent from escaping/killing you
-Helping teammates escape using rupture/scream
-Being a master jungler
-Not trying too hard to tank until late
You should do well and become a Pro'Gath user.
Ghost: Can use ghost as a get away along with Cleanse so you don't lose precious stacks of Feast/Items, or a rundown utility to catch someone with a finisher, either way its a key spell for Pro'gath.
Ignite: Used to help finish kills when your spells are tapped, can be an advantage early in games when you have rupture poked them enough to Ignite for kill.
Cleanse: Used with Ghost in a getaway situation to prevent you from dying early or losing stacks in the later stages of a game, must have for a team with lots of slows, snares, stuns, etc. Was used before season one, but now with the nerf to the duration it seems to have lost it luster to me.
Use 9/0/21 no need for any in Defense so leave that skill tree at zero. Recieve Magic Penetration, Cooldown Reduction, and Mana Regen, enough from runes/mastery/items and your spells will be hitting hard to three shot squishes from any health with feast. Use the point in utility called Haste in the top tier to move down to the next one, helps your ghost which is the main summoner spell for this build.
Some prefer to gain more gold using greed, however it is just a grain of salt and usually not noticeable with Cho'gath. Have also tested the viability of the neutral monster buff, and found it is also not worth spending two points especially if your into ending games quickly.
Basic caster rune setup most Annie/Ryze/Viegar types use it. Swapped AP for SPen quints after patch and nerf to cleanse and not getting it from boots anymore.
Items and tips coming!
Credit toMobafire, lonith(member), linabx(member), and those who have fed me in game ^^.