When fighting Covenant or Floods, try to spot which weapon is the most available. In some missions, for example, it might be the Plasma Rifle and the human Pistol. Once you've spotted these, make use of them. If you arrive on a mission and find a weapon but soon run out of ammo (or battery life) and can't find any, it could put you in danger. Especially in the heat of a battle. So keep in mind which weapon comes up the most in which mission so you can make great use of them.

Another tip, when battling (especially the Floods on 343 Guilty Spark) don't make use of all your grenades. If you see you can't get a clear toss, then don't waste one. Grenades are most rare when needed.