Ok, I'm a total newbie on this site, so bear with me on this. I'm going to share my strategies and tips on this guide. Since multiplayer on this game is like a ghost town, it's going to revolve around the campaign and some quick play levels, tips on getting achievements, and a couple of Survival Mode tips. A lot of these tips are pretty much common sense for all FPS's.

Basics

• As in all First Person Shooters, aim for the head anytime you can, not the torso or limbs. Shooting at the head kills the enemy much easier, and I found that just one shot from even a machine gun will successfully kill them if you're not too far away and have a good aim at the head.
• Don't stand out in the open! The enemy will find you much quicker, and if you're playing on Veteran, they will find you and kill you instantly. Hide and crouch behind objects whenever you can, and even though crouching gets you closer to the Sitting Duck Achievement, it's worth it over getting a ridiculous amount of deaths!
• Don't count on your NPC allies to shoot everybody for you. In fact, don't count on them to do much of anything, except open doors for you and tell you what to do next. The only time they will do a good job killing the enemies is if you're playing the challenge and you have to kill the enemies yourself!
• Be careful when trying to RPG a tank. You have to take them down with the RPG twice and still remain behind whatever's guarding you behind the armor. If you move too far away from your hiding spot, the tank will shoot you down quickly. You should only move so far that you can see just a sliver of the tank.
• Be familiar with the map of the level you're on. When you're playing a level for the first time, enemies can pop out when you least expect it if you're rushing through the level, and kill you when you're caught off guard. Remember what triggers the enemies to come out, so you can be ready to kill them fast.
• When your screen is flashing red from losing health, get to cover quick, because another shot will quickly kill you.
• The best way to not die is kill enemies before they kill you!
• Reload your clip whenever you're not shooting people so you won't have to when you're in the middle of killing people, because that second it takes to reload gives your enemy a broad window of oppurtunity to kill you.
• If you don't want to be stuck with that crappy pistol when your main gun runs out, pick up a gun that's dropped a lot by your enemies and walk over them to get ammo for the gun.

ADS Tips

• ADS is better when you're in a fixed position, not moving around.
• If your gun is something like a pistol or the AK-47 iron sight wise, then the iron sights aren't very helpful, so you're better off just aiming from the hip.
• The ideal guns to use for ADS is ones with the Red Dot Sight, or a circular iron sight where you can see the enemies clearly through it.
• ADS is most helpful for headshots or targets far away from you.
• You move around slower when you're on ADS, so it's better not to use it if you're moving around and/or surrounded by enemies.

Sniping Tips

• When you're required to use the sniper rifle in the campaign, it's when you're surrounded by enemies. Be quick when aiming, so you don't get shot first.
• As with other guns, aim for the head! In the easier gameplay modes, it takes one shot anywhere to kill an enemy, but on Veteran, it takes two shots to kill an enemy if it's not a clear headshot.
• Be sure that when you're sniping you're partially hidden, because if an enemy is shooting at you, it will be too late to find which one is shooting at you and kill them.
• Multiple shots in quick succesion is a bad choice for this gun, because the recoil on the scope is high and noticable.
• Don't aim from the hip. There's no crosshairs, and it's obviously not what the sniper rifle is for.
• This rifle is best for distance.
• Take every oppurtunity when there's enemies in the process of respawning to reload your clip as it takes time to reload it.
• When moving your scope, do everything you can to be swift and precise when you aim. Don't even think about shooting if you're sure that your not on your target, even if you're getting shot at! It will take longer to re-aim because of the recoil and your missed shot.
• Moving the scope around using the touchpad is tricky, so use small movements if you can't get the aim right, or just aim for the torso for now if you can't get exactly on the target's head.

Grenade Tips

• The grenade is best for when you're being stealthy and need to kill enemies you can't see but you don't want to risk getting in their line of fire.
• Use the radar on the touch screen to get an idea on where to throw the grenade.
• Throw the grenade close to the enemy, but not right in front of them. Throw it around them, because if they can't see you then most of the time they won't see it if it's not right in front of them.
• Throwing back a grenade: If it lands in front of you, use the hand icon to pick it up. If you pick it up right away, let it cook for a second or two, then throw it back. Do not hold it in your hand for more than two seconds! If you do, it will blow up in your hand or in the air when it's still close to you! If the grenade lands away from you, don't look for it! Get away from it because it will most likely blow up while you're looking for it or blow up before you can throw it back.
• When grenades are thrown at you in survival mode: Just get them away from you fast, because wasting time cooking them and aiming them just lets the other grenades thrown at you blow up before you can get them. When grenades get thrown at you in survival mode, multiple will be thrown at you at the same time by different enemies if you're hiding.
• Flashbang tip: Don't look at it when thrown. Make sure your enemy is close to it when thrown.
• If the grenade hits the wall and bounces back to you, get away from it quickly if the hand icon doesn't appear when it lands on the floor.

RPG Tips

• Make sure you're in an open area. If you aim at a wall close to you, you'll just blow yourself up.
• Aim at a wall behind your enemies, or directly at your enemies.
• Best for killing multiple enemies and taking out tanks when told to in the campaign.
• For the first Survival Mode level, get the RPG if it's there. It's helpful for taking out clusters of enemies.

Achievement Tips

Making Quick Work

• Sniping and aiming for the heads will quickly get you this achievement.
• Using a gun with Red Dot Sight helps also.
• When you're hiding from an enemy, quickly aim at the head.
• When the magazine on your main gun is out and you're stuck with that lame pistol, kill your enemies quickly by aiming at the head!

Up Close and Personal

• Some enemies like to sneak up on you and get all up in your face. So take them out quickly before they get you first.
• Don't run up to enemies. Most of the time they get you first.

The Hard Way

• Just stick with that dissapointing pistol when your main gun runs out to get this achievement.

Throught the Crosshairs

• Play the levels that require you to use the sniper in quick play to get this one.

Casual Gamer/Deja Vu

• Replay all your favorite levels in quick play as many times as you can.

At Your Command

• Complete the campaign on all difficulties, unlock game modes, play some quick play levels, get as many achievements as you can, get all the collectibles you can, and before you know it, you're a general!

Well Rounded Soldier

• Test out guns enemies drop to see if you like them, and try killing at least one enemy if you notice you're carrying a new gun in the campaign.

Pack Rat

• Collectibles can be tricky. They tend to find you when you least expect it. Just search every nook and cranny in each level, especially corners, because that's where they tend to hide. And check and see how many collectibles you got when you're looking for a level to play in Quick Play, and play that level again if you didn't get all the collectibles.

Whole Lotta Bullets

• Completing the campaign once will usually easily get you this achievement.

Well there you have it. Any other tips you have tell me and I'll credit you and put it on here!