HT Games
Movement and Cover
Master Chief's shield allows him to withstand a good deal of punishment for a short amount of time, but you should not rely upon it as the first line of defense. A far more effective tactic is to avoid getting hit at all. Use physical objects in the world to block incoming fire whenever possible. When out in the open, constantly be on the move. Strafe to the side to avoid incoming fire and move from cover to cover. The longer you stay out in the open, the greater your chance of being hit.
Weapons
At easier difficulty levels, virtually every weapon can effectively defeat enemies. At higher difficulty settings, especially legendary, weapon choice means the difference between life and death. A carbine or battle rifle should be in your inventory at all times. These weapons have extremely high accuracy and can drop enemies quickly with head shots. When you face opponents using energy shields, an overcharged plasma pistol can drop the shield in a single shot. Dual-wielding it with a magnum pistol will allow an immediate fatal head shot the second the shield drops. Some weapons are best suited for specific situations, such as a sniper rifle for long distance combat or a rocket launcher when facing vehicles, but these weapons have limited ammo and have little usage outside of their specific situational strengths.
Enemies
Knowing the enemy is one of the keys to staying alive in the game. Every enemy in the game has a weak spot -- for most of them it is the head. Hunters, on the other hand, are heavily armored save for on their back. Head shots on flood enemies are useless; aim for the flood infection form embedded in the host's chest. Always aim for the weak point. Any other shots will exhaust ammo supplies and take more time to defeat the enemy, giving the enemy more time to return fire.
Strategy
Patience is a virtue. While it may be tempting to charge head first into combat with guns blazing, it is a far better strategy to hang back and plan the attack. Whenever possible, utilize divide-and-conquer tactics. Pick off stray enemies and lure smaller groups away from the main force. Prioritize targets; choose the smallest and easiest-to-kill enemies first. Smaller enemies don't pose as much of a threat on their own but can be deadly when backing up other foes. Each enemy that you kill quickly means less incoming fire, making it easier to focus on the larger and more difficult foes.