Easy should be more randomized.
Normal should have the AI attack based on units in range, and if it can finish them off. (possibly a smaller depth search of moves 1-2 deep).
Hard should have depth search of future moves.
Impossible should be a trained AI against other AIs.
Maybe additional mechanics also get added based on the tier of difficulty. For example: fatigue isn't a thing in easy or normal but it is in hard and impossible mode.
Easy, Normal, Hard, Impossible
Easy should be more randomized. Normal should have the AI attack based on units in range, and if it can finish them off. (possibly a smaller depth search of moves 1-2 deep). Hard should have depth search of future moves. Impossible should be a trained AI against other AIs.