Go to the main menu and type ARMOR. You will have unlimited health.
On the press start screen (seen before the main menu press the following key configuration in order). If the cheat is entered successfully you should hear a gun shot.
Coverpoint (Space), Right (D), Right (D), Left (A), Left (A), Change Weapon (Q) - Unlock Everything
PC in brackets are the default controls. If this doesn't try it with cursor keys for left and right.
Terminate Mendoza with AK47 in ACT 05: Finishing Business. Move left and right and use mostly iron body as a cover rather than sand bags. When Mendozas gun pointed towards your partner to save him target Mendoza body and start firing to reward his attention towards you. Note when your partner would be on left side you must be on right side and vice versa.