Grenades, quite possibly the best weapon in FFA games, and probably any other game mode as well. They leave the enemy weak so that you can quickly finish them off with just about any gun. While the 1337 gamers use grenades, it can also help a noob out as well.
To start off you should know the attributes of the different grenades. They each do different things, and if you cycle between your grenades effectively for different situations you will find out just how helpful they can be.
The frag grenade is the most common grenade on the map. For this reason they cannot make them overly powerful. In order to kill somebody who has full shields they have to practically be standing right on top of these. Why are they helpful then? If they don’t have their full shields you can easily finish them off with these, or steal kills easily. Lob these into a group of people and you are sure to kill one or weaken both leading to an easy double kill. These are also helpful when you are getting BRd. You will take down your opponents shields and they will be vulnerable on the first shot.
The sticky grenade is the favorite grenade of almost everybody, and the only other grenade to appear in the whole Halo trilogy. The idea of a sticky grenade is pretty simple, hit your opponent with one of these and it will stick to them. If it is stuck on them it is an automatic death. This grenade can be used as last ditch efforts as it will usually kill you too if you are to close to them. Simply hit your opponent and start backing away, there is no point in shooting them if you hit them. These grenades are also effective if you use them to attach to a vehicle to blow it up.
The spike grenade is similar to the sticky grenade, only its purpose is for blowing up vehicles. Sticky grenades will need two to take out a vehicle, and a spike grenade will take out that same vehicle in one hit. It can also be used to “stick” opponents as well. A great tip I found out is that throwing one of these inside a bubble shield is an instant death for all those inside of it. The spikes bounce around until they find a person in a bubble shield. These are pretty useless if you just throw them in the open to cycle to a different grenade for that purpose.
The key is knowing when to use which of the grenades. When you are at a distance trying to steal a kill or weaken an unsuspecting opponent use the frag. When you are up close and about ready to die, or you are trying to get an easy kill use the sticky or spike. You can cycle through your grenades by pressing your left bumper and you will see the box around the grenade icons move.
Grenades are your friend. If you can learn to use them effectively you can become a pretty decent player in free for alls, and just about any other game on halo 3. This is something you truly need to learn.