You mean an AoS? Like DotA Allstars?
There are various libraries and demos for such AI. Obviously it will not be that good (humans will likely reach 100% win rate) but it will at least be better than nothing (could still give newbies a run for their money).
The basic idea is that you have a decision function called every second odd and a tactical function called multiple times per second. The decision function determines what the AI is to do (eg, what lane, if it should retreat, if it should buy items etc). The tactical function is how it responds to what is nearby (eg what abilities to cast, what allies to heal, what abilities to dodge etc).
You will want some helper functions to keep track of the overall known state of the arena. For example enemy hero locations, ally locations, tactical "objective" locations etc. These are mostly used by the decision function to determine what action should be taken. For example trying to match up enemies in lane distribution, or running to an ally if threatened, or even organizing a team assault.
Certain customizable elements like talents, abilities and items are also managed by the decision function. These should come from pre-built tables of loadouts that are effective to use. Human players will need to learn all the options and the most effective choices however the AI can be hard-coded such knowledge. In the case of talents and abilities 1-2 orders per hero would suffice. In the case of items you could have a dozen odd generic sequences (items for tanks, items for assassins, items for pushers etc) which you assign to classes and then give each hero one or more classes to choose from. Which actual sequence the AI uses can be chosen during AI initialization using a random integer. In actual team competitive games there is usually some form of planning that goes on but such complexity is unnecessary until your AI is working and people are complaining that it is too easy.
