actually, there is a simple way to finish campaign Sniper Mission, here is the trick:
- Protect the Sniper with Machine Gun Tower (MGT), keep his life full. dont move him forward too far until the Alien Commander is coming. first, u must finish the alien army with MGT, when just a little alien army remained and they are the weakest, slowly move the sniper forward. test the Sniper's shoot range. when the range with the commander is close enough, then shoot him !! (select Sniper/press Z then click the Commander). DARRR!! the commander is died and you win...
proceed to the next level... :)
 "My \"Dear\" Boss"){kind=icon}
 "Squadron Auriga"){kind=icon}