Okay
First off all: Say what all specials of parts do.
Second, give a bit more tutorial, like how to defeat air units and mech's (You could discover it, but it get's a bit boring)
third: A bestiary of Enemy's
-Always have a instant-to-upgrade button after a level
-When buying a new item, see what (dis)advantages you get
-Upgrade possiblities of strength, hp and focus, but regular and Not buying a new part.