This is a tactic I've seen some very smart teammates use in Gambit and I have to say, it really seems to being working. Don't kill the Wizards, don't use any Power Weapons, Supers or the IKELOS shotty. Simply sit in the back of the spawn and shoot the Primeval with Auto Rifles. Slow and steady wins the race afterall. Also, remember to let the Invader kill you any chance you get.