I just played a game of Invasion: Slayer on Boneyard. I played as Elite and at the start of the game, 3 from my teammates quit. It were 6 Spartans VS 3 Elites but still we won with 100-58. It wasn't because the whole enemy team was having breakfast and forgot turning off their console. Nope. They were all active. We didn't even had to camp to win. It was because of teamwork: One took the role of marksman and he respawned in The Bulkheads. Another one took the role of distractor/Grenadier and respawned in the Hold. I took the role of Assasin and respawned in the Stern Scaffold. Everybody had a different spwn location so we could flank them. The enemy all respawned in the same place and sticked close to eachother. They were easily picked off by the Grenadier, Assasinated by me when jumping off the platform and the leftovers were Needlerifled. Since they were larger in numbers they could conquer all territories but we didn't care. We ended up stealing their weapons. Muhahaha! We advanced to the second phase after 2:30 minutes since we had so many targets and they were so dumb. So what i'm trying to say: It is possible to win if you are outnumbered.
Its possible with teamwork, and if the other team isn't that good. If the other team is smart, and takes their sniper rifles and rocket launchers that they get, they'd be completely unstoppable. It also helps if the team you're stealing weapons from is humans, because frankly covy weapons suck, the only problem is if they get a tank, but that can easily prepared for.
Tanks? I shoot an overcharged PP shot, The Grenadier and i throw 2 Plasma Grenades each at it and there ya go. Bai Tank.