If you position your towers with overlapping firing ranges it will help them dispatch enemies quickly without taking too much damage on their own.

If you have a stray tower that's seeing a lot of action, consider upgrading its height (number of guns) quickly. that alone will give you twice the bullets to come from the same tower.

During the "Coin Frenzy!" at the end of every wave, be sure to tap around with two (or more) fingers as the frenzy ends. It's in your interest to collect as many coins as possible and use them for future tower upgrades.