Great concept. Patience and careful use of your weapons/movements is key for getting a perfect on several of the levels. One annoying bug that I keep getting hit with though. Every so often, I get damaged by my own gas ... right after I drop it. The same plane dropping it keeps going forward but when the next round starts, I get damaged. 3/5 for that.
@snakeimposter - lvl four is a pain. i got it by carefully placing four basic gunners (two on the left and two on the right side of the map), upgrade them once. upgrade a right one again. and then upgrade them as you can until maxed. after that get some electro's going on the right.