I was doing great, destroying planets in a single hit, taking down everything without trouble. Then I went to a new sector where every planet had three times as many enemies which all took multiple hits to defeat. Now Bun Bun is no more.
Even though there were lots of explanations for how things worked, I didn't really know what the optimal strategies were. I mostly sunk all my points into stats after every planet and never used the special abilities. Somehow those stat gains weren't enough after a certain point, so perhaps I missed something important.