TIP: Let the final few bugs in a level attack you. Move your gunners away from them and just keep the repair men at that wall. The repairers will get lots of xp this way, you can just leave the game running and do something else. When you come back, they will have leveled up insanely and you can sell them and repeat the process with new repair guys.
The battle system is completely flawed. If you want it to be a rock-paper-scissors kind of deal instead of pure luck, you should be able to see what's coming up in advance.
I reccomend a short period (one second?) where you see what's coming and your reflexes would decide whether you get the right unit or not. You could the buy more time to react (scouts?).
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You can, using ctrl, ctrl + shift, or just shift.