Not many people will know but you can actually control when Air Fuel Grenades go off, and you can use this as a form of trap when playing both offline and online.

To do so, when you press Circle to throw the grenade, simply don't let go. Then when the enemy walks over the area where the grenade is, you can simply let go of circle and the grenade will go off.

You have about 20-30 seconds however before the grenade goes off by itself, so use the extra 20-30 seconds that you get to kill as many enemies as you can.

This might seem kinda common sense, but do not kill friendlies (AKA, humans) they help you out, and can turn the tide of a battle.

This might seem like common sense, but TAKE COVER!

Concrete barriers are destroyable, so try not to hide behind them too often, but mounds of dirt are good hiding places.

Be careful not to hide behind vehicles of any sort because they cause explosion which would lead you to your inevitable death.
You could hide behind cars that already exploded though, but that's a lame idea, unless you don't have any choice.

To do the Bullseye Trap you simply need to fire a Bullseye Tag onto the ground (by pressing L1), and be sure to keep L1 held down. Then simply fire a couple of shots, and you will see that the shots will start to twirl around the Tag. At this point you can let go of L1.

You can now direct where all those bullets go by pointing to the target and pressing L1 (you wont use another tag). If you've used enough bullets and the target is an enemy, it will kill it. This can be much more accurate and useful, especially when you don't always have the target in sight.

Here are all the online ranks and what you unlock with them.

Level 1: Private
-- US Soldier Body
-- Head 1
-- Head 2
-- Small Helmet
-- American Standard Uniform
-- American Dirty Uniform

Level 4: Private First Class
-- UK Soldier Body
-- British Green Uniform
-- Canteen 2

Level 7: Corporal
-- British Brick Uniform
-- Grenade Pouch
-- Leather Packs

Level 10: Sergeant
-- Pouches
-- Medic Helmet 2
-- Head 3

Level 13: Gunnery Sergeant
-- Beret
-- American Camouflage Uniform

Level 16: Staff Sergeant
-- British Goggles
-- Radio
-- Binoculars

Level 19: 1st Sergeant
-- Commando Body
-- Commando Standard Uniform
-- Head 4

Level 22: Sergeant Major
-- Commando Camouflage Uniform
-- Belt Knife
-- Backpack

Level 25: Command Sergeant Major
-- Chest Pouches
-- Belt Pouches
-- Head 5

Level 28: Sgt. Major of the Army
-- Medic Helmet
-- Bedroll 1
-- Chute Pack

Level 31: Lieutenant
-- Head 6
-- British Desert Uniform
-- VTOL Pilot Skin

Level 34: Captain
-- Bedroll 2
-- Pouches
-- Pouch (Fanny Pack)

Level 37: Major
-- Head 7
-- British Helmet
-- Backpack

Level 40: Lieutenant Colonel
-- Commando Leather Uniform
-- Chest Radio
-- Shirtless Skin

Level 43: Colonel
-- Camouflage Helmet
-- Leather Backpack
-- Back Holster

Level 46: Brigadier General
-- Head 8
-- Canteen 1
-- Radio

Level 49: Major General
-- Winters Skin
-- American Advanced Uniform
-- Hip Bag

Level 52: Lieutenant General
-- Head 9
-- American Helmet
-- Commando Arctic Uniform
-- Canteen

Level 55: General
-- Cartwright Skin
-- Leather Pouch
-- Canteen

Level 58: Supreme Commander
-- British Camouflage Uniform
-- Knife
-- Head 10

Level 60: Supreme Commander [3 PIPS]
-- Nathan Hale Skin
-- Belt Packs
-- Shovel
-- Backpack

When In A tank (M-12 Sabertooth)
Don't keep shooting just shoot one at a time and it is easy if you don't switch to the machine gun
just keep the cannon on!
You may as well just run over the chimera as well since they die!

Be careful not to leave your partner when riding the elevator, sure some would take the two of you automatically but some elevators won't.
My character went up the second floor and I clumsily left my partner.
I tried to go down again to pick him up but the elevator won't go down anymore, sorta stuck.
So we just both killed ourselves.

After you've successfully finished the hard mode, go to options and choose Cinematics. Look for the movie list entitled "Stranger Than Fission". You may watch there a funny last scene from the last battle of the game.

In Manchester near the bridge by a truck or bus there is a dead british soldier who looks like hes looking at his watch.