Just a bunch of tips to try in Multiplayer.

Grab body armor as often as possible. Even if you still have half body armor left, this will make you last a bit longer, and it will make it harder for opponents to grab body armor.

Try to rush to opponents after they are killed. This can prevent them from grabbing the best weapon, and from getting body armor.

Don't be predictable. Sometimes wait for opponents to come to you, sometimes ambush them, and other times pretend to wait, then ambush. This will keep your opponents guessing.

Be a third (or fourth) man into a battle. Your opponents have already partially worn themselves down, and you can jump in from nowhere and get two (or three) easy kills. This tactic is cheap, but it works.

If playing with Proximity Mines, plant them at the starting points. You can rack up easy kills, but just hope that you don't end up there.

Again, ambush killed opponents at their starting points.

Don't grab body armor!

Running away unarmed leaves you vulnerable for a bullet in the back. So if you are unarmed, try to get behind the person, up close, crouch, and slap. This works often, and really annoys opponents. They sometimes panic, need to reload, and get killed by a single slap.

Watch for opponents limbs. If they are hiding behind pillars, walls, or doors, all you need to see is a small part of their body. Even a shot to the toes, or hands can kill.

Shoot their guns! Lots of times, opponents hands and guns stick out through doors. Even if you shoot their gun, they die. This is really effective with Sniper Rifles, which have very long barrels.

If you are peeking out and trading shots with an opponent, it is better to have the opening to see your opponent on your right. All characters are right handed, so that means that your gun will be pointing out with as little of your body as a target as possible. If you shooting your opponent from the left, you have to expose your whole left side before taking a shot.

With the Gun

Try not to get into open room fights. You are just firing single shots while you are being blasted. Jumping out from behind doors and pillars is a good tactic.

If possible, take on foes one on one. You can't always avoid being double teamed, but if you are, you will definitely take damage. Your gun is stronger, but the others shoot faster, and don't need to be reloaded after every single shot.

Try to be in areas where you can escape if you are double teamed. When they come after you, one is usually a bit ahead of the rest. If you take him out, your chances of getting the rest are better, especially in three player.

Although body armor does nothing against the Golden Gun, if you have armor when you pick up the gun, you will keep it. This is a good strategy, because you can't get armor while you're carrying the Golden Gun.

Ambush the Golden Gun holder and aim high. Dodge from side to side, making yourself harder to hit. Remember, head shots do the most damage.

Try to double team the Golden Gun holder. This ensures that he will take at least a few hits. Unless, of course he is very quick and never misses, in that case you're screwed.

If you are really cheap, you can take the Golden Bullets from where they appear. This prevents the Gun holder from getting them, and he'll run out, especially if he has just gained possession, he'll only have a few bullets before he runs out and needs more. 

It's a good strategy to be with your teammate as often as possible. Try to attack a lone member of the other team together. Each of you shoot at him from different directions, like his front and his side. It also works shooting front and back, but there is more chance that you will hit your partner.

2 Vs 1 and 3 Vs 1 are easy for the team, and hard for the lone agent. Some of the tactics I have listed below are dirty, cheap, and dishonorable. Use them if you are like the person described above, or if the lone agent is truly a cocky 007 expert that needs a good beating.

If you are on the team, you are most likely against the best player. Try to take him on two (or three) at a time. This will ensure that shots are hitting him, while he can only hit one of you.

Grab the body armor if he is anywhere near it. If your team does this, it's an easy battle.

Spread out immediately after he gets killed. This provides more of a chance that someone will be near him when he appears. Make sure he doesn't get a good weapon or body armor.

Try, if possible, to avoid being ganged up on. This can be tough, especially in 3 Vs 1. It can be a good strategy to hide in a room with only one door, so you can't be caught from behind. A good example is the bathroom in the Facility. If you are a good player, you probably won't get hit as often as some of the others, making you live a bit longer. But if you are hiding in the Facility bathroom and it's 3 Vs 1, the other players will probably take turns grabbing the nearby body armor. But if it's only 2 Vs 1, when one of them leaves, finish off the other guy before he comes back. Sometimes this is easier said than done.

With the Flag

Keep running at all times, don't stop. Before you pick up the flag, be unarmed and hold Z. If you time it right, you can keep holding Z and slap. It doesn't do much, but at least you're not helpless.

Close doors behind you. You can even stay behind a door, and keep it shut. This will really piss the other players off, and once they catch you, you'll get it good.

Without the Flag

Grab Body Armor. You can't get it once you grab the flag, so get it before you pick it up. If playing with 2 or more others, they'll all be going for the guy with the flag, and not for you.

Watch the flag carriers health, and ambush him at the end, running right up to him, hopefully being the one who grabs the flag.

WEAPONS:

In each Multiplayer Match, there will be 4 types of guns (there are only three in Pistols, only Knives in Throwing Knives, and nothing in Slappers. There is the selected weapon, along with a second weapon, a medium weapon, and a bad weapon, always a pistol.