I modified my previous design and was able to get 3892 and 18 lives left. I didn't make a dent in the last flying boss, and I was able to split up the swarm boss, kill one of the splits, and nearly kill the other one. I basically used my last design, and this time focused on upgrading all the pelett towers in the flying creeps' path as much as possible. I ended up with about 6 sniper towers which did a whole lot of damage.

*edit*
Now I wish this were a standalone game. I hate having to go to the website to play it. I'd pay a little for it, too. I also wish I could cheat for fun. I'd love to see those last bosses get past a screen-full of maxed out towers