This is a nice game, but it's really hard (or maybe I'm just a noob)
It's just a matter of figuring out the optimal paths/builds. But don't worry about it too much, since I'm making a meta-upgrade system you will be able to get stronger outside of the main game loop and eventually reach higher rounds.
For now, I would say that going Burst/LavaPool, Orbital/Ice or Statik/Vamp are the most broken builds with the latter one being the easiest to pull off