Welcome to a question cosplayers have been asking since the first guy took up the first costume and stood under the heat, cursing the harsh glare of the sun as he melted into a puddle of sweat and gross.
There are as many opinions on how to do this as there are people willing to offer one for you. It all really depends on how much space you have under the armour itself, and how much you're willing to spend, but the generic go-to answer is usually going to be some sort of cool vest or variant thereof. The main issue is going to be keeping your core temperature down, so cooling your head and torso are key.
Other than that, the best I can suggest is nothing more complex than taking frequent breaks and sucking down plenty of good, old-fashioned two-part hydrogen, one-part oxygen while languishing under the AC. Most conventions are good enough to have some sort of 'suiter lounge' where you and your handler can go to take breaks now and then.