I'll admit something. I went through the first 6 missions without upgrading anything. Then I was confused/getting my ass kicked in mission 7 and decided to look at the upgrade menu. Holy crap at the options you've got health and move wise, many of which turned about to very helpful in that boss and everything I've gotten up to at the completion of mission 10.
Mission 10 in particular is making me a real fan of this Suda 51 guy.as the entire levels boss. Pure craziness in the best way possible."Tommy" the tank engine
It's funny because I did mission 7 last night and was amazed that if you weren't upgrading properly how lightly fucked a person might end up. That's a new one for a Grasshopper game.