The key to getting far on the crater  map is to get the covenant as they spawn.

If playing with multiple people, have one person man the turret overlooking the plaza, and the remaining players equip themselves with the rocket launcher and/or sniper rifle.

Have the turret man concentrate his fire on the enemies either leaving the drop ships or those rushing through the shield barriers.

Protect the turret at all costs

If the man on the turret is near death, switch him with some one else so the turret is not destroyed

The turret only respawns once every wave, so keep it alive for as long as possible

If you feel lucky, break off the turret right as the round is ending, so you will have a man running around with a turret as well

Keep the enemies from reaching the turret man's flanks, as he can not aim that far to his sides