Simply put that it is very easy to overpower any invader with the following set up for the marines:
Marine 1: Officer, Special ops (x1 or x2 depending on how many marines there are)
Marine 2: Tough, Survival ops (x1 or x2)
Marine 3: Sniper, Marksman (x1 or x2)
Alternate: Killer Instinct, Ground Assault, (Medic)
the top one is probably the best, but the bottom one is also juicy too!