I would suggest using bennygreens strategy for all the levels except the last one. I fused Bennygreens and krolans advices together and added something, you really want to use the lightning spell on the last level especially on those elementals and whoopsiedaisy it also damages crowds not just one enemy. The drakes are easily eliminated with the fireball iceball strategy. You can easily manage to do all this and easily beat the last wave. Oh and also use Bennygreens trap formation.