Get good teammates, counter-fit everything they send (e.c.: covenant anti infantry- jackal, anti vehicle- hunters, anti air- vampire, UNSC; anti infantry- flamethrowers, anti vehicle- cobra, anti air- wolverine.) Get a 3v3 team, have someone build entire covenant army of jackals, have the other 2 players build an army of cobras, the last player build entire army of wolverines. Leaders don't matter, whichever you prefer. For UNSC fully upgrade the 3 spartans you can get and use them to support as well, for covenant recommended leader is regret and arbiter.
(This is an idea I've had but never used.)