Try having 5 (4?) catapaults at once, stack to 7, then only refresh the stack when it has 2 seconds to go, it's the ticks that do the damage. If you burst to 10 stacks, you get not much dmg + a 10 stack dot, if you go slowly, you get lots of increasingly powerful dots + a 10 stack dot.

I did it with about 11 chars and just stacked all in the one spot. I also prepped with shooting down barrels and had the catapault guys targetting blue stuff, when i got to 7 stack i had them refresh the energy then manually went to each one and selected a new barrel. I also had a mate help for the interupt and shooting adds.

And just pray he goes for the char in a non-catapault second lol.

Been a while since I have played but this should work with some luck.

3 towers you can just burst down pretty sure.