At the main menu, press L(2), R(2), B(2), Y(2), X(2). There is no confirmation message. Start a game, then pause and enter the options screen. The "Cheats" option will be available at the options menu.
Successfully complete the game on the hard difficulty setting to unlock the extreme difficulty setting.
If one of your teammates falls to the ground, throw a smoke grenade near him. Then, get your health packet out and heal him.
Use Jones to run up to the vehicle or tank. Lay an anti-tank mine next to it, in turn sacrificing jones Temporarily to take out the vehicle or tank.
Use Jones to run close to the tank, then plant a C4. Throw a smoke grenade on the tank, run for the nearest cover, then blow it up.
Have one of your team members run by the tank to distract it. Then, run up to the tank, plant C4, and blow it up (risking your life). You can heal him with the other team member.
Tanks are weak in the following locations: their rear; in front, in between the armor over the tracks; and where the gun barrel meets the turret.