Agreed with Jzyehoshua. In general the game ain't bad - a good ol' "shoot 'em all" slaughter, no thinking needed, just your reaction.
Upgrade both guns by turns. By the wave 6 you should have two gattlings and two plasmas. Your strategy should be the following:
1) Keep shooting wether there are enemies on the screen or not: try to cover the entire area with fire, so when they come out they already get some.
2) When you see a robot (first you actually see his shadow) focus on him. Robots should be your main priority, since if you let them stack up they will vanish your base in no time.
3) The base can stand for a while against infantry, but if there are no robots still clean them up with gattling. Still though your priority should be robots, buggies and only then infantry
4) Use hotkeys for upgrade, clicking a button wastes your time.
BTW: you can pause the game with [SPACE]
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Good point here :) We will appreciate if you can lift our rating up little bit ;)