Well, I took up some quick research and this is probably the best I can come up with. So Lets start off with the ability itself.
Cho'gath's Ult
Ravenously feed on an enemy, dealing 300/475/650 (+70% Ability Power) true damage to champions
True damage is a pain. Back then, people relied on the "dodge" mechanic to avoid them. But with it's removal from the game beside champion abilities and few other exceptional runes that I heard that stayed in the game. Is building Life.
Here's an example post on people talking about it.
http://forums.na.leagueoflegends.com/board/showthread.php?t=1625165
Another method is to "kite" him so he is unable to ult you. So champions that can slow or is plain faster than him will have an easier time.
http://www.championcounter.com/chogath
And lucky for me on adding that, on the same page. there is a tip that back up the building life method.
"Building health can help protect against the true damage from Cho'Gath's ultimate."
As for his passive, on the same page, there a tip on it too!~
"Try to kill Cho'Gath wherever possible to avoid his Feast ability stacking too high."
Just don't let him stack up. If he does manage to farm up and get big. These items will always be useful. Depending on your champion and their team setup.

As for smite buffs. The Chilling smite can help you kite him. and challenging smite can lower his other damage and increasing your own damage.