AP Cho'gath is what i consider to be one the best mage bruisers in the game. try him out.

Pros:
knock up slow
naturally tanky after level 6
cone silence
high powered true damage ult with low cooldown
can solo top

Cons:
very slow
mana hungry early game
high learning curve

I start out with dorans ring for early game ap, mana regen and health. The mana regen is the most helpful as cho is very mana hungry. when i port back, i get either boots of swiftness if i need movespeed or boots of speed and an amplifying tome. the rest of the build is based on what i need when, but prioritizing lich bane as it makes chogath godlike.

at low levels, it can be hard to farm. yes you can drop the ocasional ruptue or scream on creeps but i usually save them for champs. at level 4, however, your vorpal spikes will be level 2 and be doing enough damage to farm pretty effectively.