My personal strategy from lvls 6-9 is:
First, upgrade all arrows, then unlock golem, dwarf and mage. Start the battle, gather loads of mana with mana pool upgrade. When you have a great pool of mana, start to summon endlessly the three creatures, youll win in a few minutes.
Hey, ive been playing one map fore an hour and a half, and there were 25 waves, i had to do something else while i waited. Great game, but it consumes my weekends!!!