I gave up after 34min in survival. You have to think what are the best spell, and not which one has higher damage.
It seens confusion is being underrated. Everytime you use level3 confusion more than half the workers just die and if the workers are dead you can't lose.
I think its possible to go on forever in survival mode if you have level3 skeletons, level3 reaper, level3 earthquake ( to damage building ), level3 mana barrel and level 3 confusion.
Just keep reaper and skeletons up all the time, spam skeletons when the big white guy atacks, and cast earthquake, mana barrel and confusion whenever possible.
no difference if you have the skill will be different bounce