Too easy, if you upgrade in the right order. Also, if you feel like it, you can just upgrade weapons, kill boss, earn $$$, die before end of level, and then kill boss all over again for endless $$$...
Loads of people are saying they're not getting the Fume King trophy after definitely defeating one. Submit a bug report to the developer so they do something about this before we lose out on 30 points from badge of the day!!
Some people don't seem to realise that there's a button at the top which says "manual timeout" which lets you end combos early if they're taking too long.