You can get a lot of level achievements by letting the last creep live by deactivating all your towers. If you have a large mana pool it will regenerate faster then the creep banishing costs. Just do something else in the meantime while your mana replenishes.
Too easy and too twitchy at the bosses (which makes it difficult..)
Suggestion: different weapons, machine gun for fish mobs, flamethrower(under water, but what the heck), active shield at the cost of fuel, etc