From the main menu go into the extras menu and using either the control stick or the D-pad input the following codes:

Code - Effect :

Down, Left, Up, Left, Right, Down, Left, Up - All Multiplayer Skins
Right, Left, Right, Left, Down, Down, Up, Up - Paintball Mode

Complete all stages on Hard difficulty

When you have a Harpoon and are trying to kill someone else with it, shoot the ground around their feet. This is much easier then trying to shoot the person's body. You may even kill more than one. Note: Do not try this with other weapons such as pistols, machine guns, commandos, etc.

When your opponent uses his or her shield, hit them with your stun power. They will be unable to hack out of it because their shield drains all of their power. Then, pick them up and throw them. When your opponent uses MRI Vision, use your shield. You will become invisible to them. Try different combinations of weapons to see which works best for your fighting style.

Down, Left, Up, Left, Right, Down, Left, Up
Right, Left, Right, Left, Down, Down, Up, Up
Down, Right, Down, Right, Down, Up, Down, Down, Right, Down, Right, Down, Up, Down, Left
Down, Right, Down, Right, Up, Down, Up, Left
Down, Right, Down, Right, Up, Down, Up
Left, Down, Up, Right, Up, Right, Left, Down
R,R,R,L,R,R,L,R.
L,R,L,L,L,R,R,L
L, L, R, R, R, R, L, L
L, R, X, Y, X, Y, L, R, X, X, Y, Y, Z

To find the golden gun you must be in the Hong Kong level after Dr. No's helicopter attacks the building(after you open the sliver case). Kill everyone on that level of the building but DON'T GO THROUGH THE HOLE TO PROCEED ONWARDS. Instead to back to where the sliver case was and look back the way you came. Go to your left and crouch down. You should be able to go through the wall(it's right next to some fire). Go straight through this hallway(still crouched) and you should be able to go through the wall to your left(next to the corner). Then you will be in a room with a cow in a boat. The golden gun is behind the cow.