An avid Halo 3 player myself, I would like to share five ways I found most effective in "pwning n00bs" at slayer, the most popular multiplayer game in Halo 3. In slayer, one's objective is to kill as many enemies while avoiding death. The team with the most kills wins.
5. Try not to stay alone, especially if you are without any special weapons or vehicles. A group of opponents can easily kill you, even if they are not as good as you.
4. Kill others from long range. Use the sniper rifle, the beam rifle, the Spartan laser, or the missile turret. Hide in a place where there is cover and kill them using the least ammunition as possible. If you shoot too many shots, the victim will eventually find out where the shots are coming from, and if you are out in the open, opponents can see you. If an opponent knows where you are, he will sneak up and surprise attack you in your hiding spot.
sniper rifle
beam rifle
The Spartan laser has a long charge and reload time, but you can hit any part of person's body to kill them. The Spartan laser, aimed at any part of a vehicle, can destroy them in one shot, unlike the sniper and beam rifle, which require you to hit the occupants' heads. 
Spartan laser
Only fire on vehicles above the horizon with the missile turret, or the missiles will just hit the ground Fire the missile turret quickly against the vehicle once you have a lock-on because opponents can hear the lock-on and evacuate the vehicle. To ensure a certain kill, fire two shots in quick succession. All vehicles, except the wraith, will be destroyed in two shots. Only use the missile turrets on vehicles from long range because it is extremely difficult to kill somebody on foot using a missile turret because there is no lock-on for people.
missile turret
3. Hide somewhere and sneak up on opponents using close combat weapons, such as the energy sword, shotgun, melee, or the gravity hammer. This strategy may seem cowardly, but nobody receives points for honor. The energy sword, gravity hammer, and shotgun will perform a one-hit KO at close range. Using the melee and the gravity hammer will blow your cover more than using the energy sword and the shotgun. The weaker melee requires that you weaken the opponent first shooting them before hitting them. The noise of the shooting will draw attention to you. The gravity hammer, which requires you to swing much earlier than the energy sword, emits a loud noise that opponents can hear.
energy sword
gravity hammer
shotgun
2. Use the vehicles. I particularly prefer the banshee, ghost, and wraith.
The banshee is a quick, agile flyer. You can boost yourself using the left trigger and shoot the primary gun using the right trigger. The alternative attack is a more powerful but less accurate fuel rod cannon using the B button. Use both attacks when taking out opponents on foot. First, use the primary gun at longer range, and then, fire the fuel rod cannon at shorter range. The explosive fuel rod canon can kill the weakened enemy even if it does not acquire a direct hit. When attacking vehicles, use the banshee's flying capabilities to your advantage. Only "splatter," or run over your opponents from behind. If they can see you trying to run them over, they will quickly move to the side and try to jump on the banshee to skyjack it. Once your banshee has been skyjacked, any smart opponent would not try to run you over and give you the same opportunity you have given him. The most probably consequence would be that somebody will destroy the banshee with the missile turret, and you would have to wait forever for it to respawn again. Sneak up behind the vehicle, and use the primary gun and the fuel rod cannon. Because the vehicle and you are both moving in the same direction, the vehicle should be easy to it. Furthermore, any vehicle that can fire, except except for the wraith and warthog if at least two people man them, can only fire from the front. This effectively eliminates you from taking damage in the banshee. Avoid taking on wraiths and warthogs if there is one person manning the turret because the turret can deal massive damage to you in the banshee. If you are heavily damaged inside your banshee, eject out from the vehicle by pressing the right bumper. Most likely, there will be no enemy underneath while you recover. Once you have recovered, get back in the banshee and start flying again. In addition, the enemy that has heavily damaged you will probably think that he has destroyed the banshee and killed you inside. Therefore, he will stop pursuing you. Remember to take your revenge on him once you have healed and are back in the banshee.
banshee
The ghost is a speedy land vehicle. Use the left trigger for a speed boost. The ghost easily flips over, so drive it mostly on smooth terrain. Sneak up on opponents from behind and run them over. Only use the gun (right trigger) if you are at close range where there is not enough room to gain enough speed or when the opponent is facing you and will jump to dodge the ghost if you come near him.
ghost
The wraith is a huge tank-like vehicle that can take massive amounts of damage. A second person manning the turret in the wraith will make it much more effective. The energy ball using the right trigger is very powerful, but it is not accurate. A person on the turret weakens the enemies before the energy ball is shot and kills opponents with its splash damage. Be sure to aim the energy ball at the ground because if the energy ball does not come in contact with anything, it will not explode and cannot deal splash damage. The energy ball is great to use on vehicles because the vehicles are much larger than enemies on foot. Be sure no enemies can get close to the wraith or they will jump on and plant a grenade or melee you. Move backwards while shooting enemies, so the enemies cannot approach you. If an opponent is too close, the energy ball has no effect on him. Avoid stopping with the wraith and try to move as fast as you can. The wraith at full speed moves faster than opponents on foot, so you can avoid them coming close to the vehicle. Always drive the wraith on smooth terrain and on wide paths. The underpowered wraith has trouble escaping once it has fallen into a hole and its weak boost does not allow it to cram through tight spaces.
wraith
And the best way is...
1. Stick together with at least 2 other people on foot and hunt down any enemies using only the assault rifle, the standard grenades, and melees. This method will greatly improve your skills on the default weapons, which can come handy in any type of pinch. In addition, this reduces your reliance on special vehicles and weapons in order to kill other players. Finally, the support of 2 other players makes it much more difficult to be killed.
P.S. I changed the song for my blog to a piece from Final Fantasy X titled "To Zanarkand" because I was bored with my old one. This song is one of the most beautiful pieces ever composed and even melodically and harmonically surpasses Pachabel's Canon in D.
