Before winning a battle, maximize the number of units that remain in the arena for the next battle.
I don't know if this is a good idea, but lately I have been trying to let Warp increase to 10 before deploying units. Its painful but being able to instantiate multi-units seems to give me a slight advantage. Might not actually be an advantage though, could be all in my head.
What units do you try to use most? Also what units are able to survive the 'big bang' between each match?