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If you can make it to the end of a fight without picking up a nearby health, do so. By being cautious and patient and using your shields, you can make it pretty far in a mission with only one health blip. The shields will replenish as long as you're not hit. If you lose all of your shields, seek shelter. Running over most enemies kills them automatically, saving you the trouble of having to mow them down. Use whatever you're driving as a battering ram. They will help out tremendously in clearing out the enemy. When driving a vehicle, pick up as many buddies to ride with you as possible. The AI is so good that usually the aliens will catch up with you, and then you'll have double the trouble. There are a bunch of areas in Halo when you can run by enemies, but it is not wise in most cases. It will stick to him, and he'll keep charging. Don't throw a Plasma Grenade at a charging enemy. Or, turn and run, seeking a position of safety to regroup. When shooting at charging enemies, keep back-pedalling until they all drop. The less weapons pointed at you, the better.
Once the weak are out of the way, go after the strong.
Focus on the small, nagging enemies first. all provide you an opportunity to fall back, reload, and not get hit! There is plenty of cover in the levels of Halo. Dodge out and shoot, then return to a position of safety. Most of the Covenant encounters in the game will be fought using some kind of cover. If you're up against the charging Flood, it's a good idea to keep a Shotgun handy. Tailor your weapon choices to the enemies you've be facing. Reloading in the midst of a firefight is a good way to lose shields and life. Any other weapon is better than an empty one. If a weapon is out of ammo, don't hang on to it. These weapons have infinite ammo and will not deplete your sacred hand-held supply. Whenever possible, use the vehicle-mounted weapons to take down enemies. If your weapons are full and you can't pick up more ammo, but there is a large pile of it on the ground, remember the spot where you saw it and return there after you've forged ahead. The less options you give the enemy, the better. Instead, attempt to toss them into groups of unsuspecting enemies, or boxed in groups of foes. Don't waste a Plasma Grenade on a single Grunt, for instance. The enemies in Halo come fast and furious, and it's good to have a weapon that can keep up. Always be sure to have a quick-firing weapon. The Rocket Laucher, however, is not, as it will blow up in your face. The Shotgun is excellent for these encounters. In close combat situations, make sure you have a good run-stopper.
This means you're going to have to be pretty savvy at knowing which weapon works best in which situation.
In Halo, you are only allowed to carry two weapons at a time: one in your pack, and one in your hands.