-MOSTLY FOR SLAYER OR TEAM SLAYER-
Ok, I thought about making this a while ago, and I think it will be very helpful.
1.ALWAYS have a secondary weapon. Spiker, plasma pistol, anything. Once you go blasting that Battle Rifle clip into somebody and start to reload, pull out that Spiker (etc.) to finish him off.
2. Don't throw Spike or Plasma grenades unless you're up close to the guy or you know you're about to die.
3. Communicate. On Team Slayer, use your mic to set up a plan. Get somebody to go get a sniper, send one or two people to scout out an area, get that guy with the weird helment to stay at base, anything.
4. Always stay close by in Team Slayer. You never know, there could be somebody in the corner. When he kills you, the other people on your team know to start blasting.
5. On Valhalla, always try to get the banshee. It might be a noobish thing to do, but your teammates will be happy you did it.
6. Try to get atleast a gunner in a warthog. If nobody gets in, don't just go driving around. Get out and grab a sniper or shotgun.
7.Try to win.
I hope these were helpful. I know it's short, but it's all I could think of.