Interesting concept and a fun game. Not too hard to figure out, but definitely becomes challenging during the later levels ("story mode") if you do not have the right upgrades/items. It's as simple as restarting the level and trying different combos after death.
The only problem I see is that I cannot seem to obtain "Upgrade Specialist." I do not have enough money on level 18, thus I cannot buy all the upgrades. I would think that completing the level (and game), then going back to do the level again would save my bought upgrades/earned cash, but it doesn't. Do I have to start over? UGH :/ . Any suggestions?