Here is an idea, change how towers deal damage

Instead of the current gradual increase in damage with each shot, why not have them deal % target's max health up to a cap, as true damage, along with some flat base damage (significant enough to pose a threat at early levels still)? The bonus %TMH damage would also increase as the game progressed. Maybe deal the targets level as the percentage (1%TMH at level 1, 18%TMH at level 18 and is capped at 18%). That way early on the bonus %TMH damage is not very high, but later on it is a serious threat where just having the tank eat 15+ of the shots is no longer an option.

Minion damage would remain unchanged from how it currently is.