***some things that may help***
-first be aware of CATCH skull!!! whenever catch is on...be very conscience of it, the four of us would yell "nades" whenever we saw an alien -blam!- his arm back! like yelling pass run in football, it makes you aware. call out fuel rods hunters, beam rifles and chieftans as soon as you spot them also.
-second, we used this technique also, in each group of two, we had a guy using ALL covie weapons, and the other used all UNSC weapons, this may not work for everyone, but doing this helps. i used all UNSC weapons and my partner used all covie and neither one of us fell short on ammo. there is plenty of UNSC ammo for one guy on each side, but not for 2. this also saves one guy on each side from having to go to the middle alot, leaves UNSC guy to cover covie weapon guy.
-one guys bait and one guy assasinate chieftans, best way.
-DONT be selfish, if you really want this achievy, focus on staying alive and keeping your buddy alive, NOT getting kills.
the only medal you should think about while playing is invincible for not dying!
-