I've burned through the medium levels. The most recent one I did I did the last 10 waves without building anything. The difficulty doesn't really ramp. Once you have 2-3 ranks of the highest level infantry, you can just burn 'em down as they come.
I don't even use cannons, there is no need.
This is a cheap ass knock off of frantic, right down to the order of the waves and the boss shapes. 1/5 for me, not favorited.
How this THIS crap get chosen for a card award? That's the only reason I would ever play it.
 "Snail Bob"){kind=icon}
 "Gravitee Wars"){kind=icon}
 "Master of Fortresses"){kind=icon}
 "4 Differences"){kind=icon}
 "Pixelvader"){kind=icon}