Halo 3 Basics

Aim High
Not only does aiming high generally offer a better field of vision, but it's important for targeting the enemy's melon as soon as his shield is depleted. It only takes a few direct hits to down a soldier.

Shield
The shield is your most valuable possession. It can absorb most attacks, but once it's depleted, you're just a shot or two away from death. However, if manage to stay out of harm's way for four seconds, it will gradually recharge itself to full potential. While four seconds may sound quick on paper, it's an eternity out on the battlefield.

Grenades: Use Them
Grenades are plentiful in most levels, so don't be afraid to use them. Remember: Most grenades won't explode until after coming to a rest on the ground, so it's important to guage where your targets going to be a second or two from now, and toss the grenade in that location. Even if they dodge it, it may throw them off enough allowing you to shoot them with ease.

Automatic Weapons: Use Short Bursts
When using an automatic weapon, it's important to fire in short bursts, instead of holding down the trigger constantly. You see, the longer the weapon fires, the less accurate it becomes. As such, we suggest squeezing the trigger for a second or two, then immediately release and reengage the trigger. What you lose in firepower, you more than make up for in accuracy.

Screw Reloading, Switch Weapons!
Possessing a secondary firearm is often crucial to success, but many people forget about it in the heat of battle. When your clip empties, slip reloading your primary gun and instead switch to your secondary weapon. This saves precious seconds; seconds that can be used for killing.

Reload that Secondary Weapon!
While it goes without saying you should reload at every available opportunity, many people neglect reloading their secondary weapon. As mentioned in the previous tip, secondary weapons are handy to have at the ready during battle, yet if its clip is running low on ammo, it'll be of little use. Whenever you have some downtime, make sure both weapons are fully reloaded.

Check your Radar
Think back to Driver's Ed; remember how they instructed you to check your mirrors every few seconds? Well, same deal with the radar. Your eyes should constantly be darting between your radar and the main field of vision.

The radar, in essence, provides a 360¿ view of your environment. Any enemy within a few meters will appear on your radar as a red dot, so long as they're moving swiftly. Of course; it's not very detailed, but it's accurate enough to pinpoint an enemy's exact location. This is why it's crucial to keep an eye on it constantly; being able to tell where an enemy is attacking from in invaluable Intel.