Reach Level 32 to unlock Veteran mode, which is a Co-op mode with much tougher enemies.
Ray Gun in "Little Resistance" mission
After the boat crashes, call in an air strike on the specified location. Run up to the beach, and move to the right. You will find a small puddle at the right side of the beach. Stand in it briefly, then move towards the middle of the beach to find a smaller puddle. Stand in that puddle briefly. Move towards the far left side of the beach. Stand briefly in the puddle in the barb wire. After several seconds, the screen will shake, and you will hear strange voices. Four lion statues will rise out of the ground with alien Ray Guns in their mouths. -From: Chris Hauserman
Bonus weapons
Successfully complete the indicated task to unlock the corresponding weapon or weapon attachment:
The Death Cards can be found at the following locations. Collect a Death Card to unlock its effect in Co-op mode.
Thunder
Mission: Semper Fi (Eight of Hearts)
Effect: Headshots cause enemies to explode
Location: At the very beginning after leaving the shack, you will see another shack to the right. The card is inside.
Hard Headed
Mission: Little Resistance (Four of Clubs)
Effect: Enemies take less bullet damage
Location: At the end of the mission before going into the last building, look to the right. The card is in a corner of bushes.
Suicide King
Mission: Hard Landing (King of Hearts)
Effect: Fire explosive pistol rounds while downed
Location: About halfway through the mission you will see a big building with a mini-trench outside. Enter the building, and go into the lower level. The card is to the right when you go into the room with the stairs.
Cold Dead Hands
Mission: Vendetta (Five of Diamonds)
Effect: Enemies take their weapons with them to the grave
Location: After the sniper section at the first part of the mission, Reznov will lead you through a building. Follow the bar around to the end to find the card.
Sticks & Stones
Mission: Their Land, Their Blood (Joker)
Effect: Your weapon arsenal consists of a knife and rocks
Location: Destroy the tanks, and go through the barn. Search the stalls to your right to find the card.
Vampire
Mission: Burn 'em Out (Queen of Hearts)
Effect: Can only recharge your health by getting kills
Location: Just before the second mortar position is a bunker to the left. The card is inside.
Flak Jacket
Mission: Relentless (Nine of Diamonds)
Effect: Enemies take less explosive damage
Location: At the end of the mission, take either the left or right path. Eventually the paths will reconnect. The card is about halfway down the path.
Body Armor
Mission: Ring of Steel (Jack of Spades)
Effect: Enemies die by headshot only
Location: Enter the asylum, the immediately go to the left. The card is in the far bottom left corner.
Undead Soldier
Mission: Eviction (Ace of Spades)
Effect: Enemies come back from the dead
Location: During the first half of the mission, just after the kitchen area, you will eventually see a room to your right that has a large hole in it. Drop down to find the card.
Painkiller
Mission: Blowtorch & Corkscrew (Ten of Clubs)
Effect: Shoot downed Co-op teammates to revive them
Location: Go to the left as you move toward the second bunker. A Japanese soldier hanging upside down is near the card.
Berserker
Mission: Breaking Point (Three of Diamonds)
Effect: Get three kills in five seconds to become Berserk
Location: Clear all four mortar pits, then go to the small shack in the south of this area. The card is inside.
Paintball
Mission: Heart of the Reich (Six of Clubs)
Effect: Paintball guns
Location: The card is in the closed-off subway entrance across the street from the start of the mission.
Victory
Mission: Downfall (Two of Spades)
Effect: Limits your HUD, turns on friendly fire, and bleed out in half the time
Location: Before you get to the theater, some of your allies will break through a door to your right. The card is past that door.
Extra ammunition
Use the following trick if run out or are running low on ammunition on any mission in Solo mode on any difficulty. Find a non-essential teammate (someone not involved in the story line). Walk up to him, and aim the reticule next to his head or torso. Knife the air, and as you do so move slightly towards them so that the reticule is over them. If done correctly, he should die and drop his weapon. Non-essential teammates will get replaced when you walk into a battle. If the teammate does not die but keeps getting back up, they are essential to the story (for example, Sgt. Roebuck and Sgt. Rezenov).