I was playing around, and made a mega mega tower with 23,256,090 damage. It was a combination of 12 splashes and the rest damage boost towers, so it wouldn't have hurt fliers. I think I could play with it and get it a little higher/
I think substituting one splash air tower would have lowered the damage some (1mil?) though.
The trick I figured out, which I'm sure other people have figured out, is that when making a Mega tower, the "new" tower has a base damage of the sum of all towers times 1.3. So you must boost your base towers as much as possible before combining; they "keep" their original damage, but then get boosted again by the surrounding towers.
One useful tip is that if you have a fully upgraded slow tower near the middle of the screen with 2 fully upgraded range boosters next to it, the entire stage gets slowed the entire time.
This isn't useful for the short stages, since you have to max out 3 towers for it. It's extremely useful for the infinite-level ones!!!
