Eliminate Mendoza with an AK47 in Act 5: "Finishing Business". Move left and right, and use mostly iron body as a cover rather thansand bags. When Mendoza's gun is pointed towards your partner, target Mendoza's body and start firing to get his attention back towards you and save your partner. When your partner is on the left side you must be on the right side and vice versa.
Press SPACE, DDAAQ at the "Press Start" screen before the main menu appears. The sound of a gun will confirm correct code entry. Alternately, press SPACE, RIGHT(2), LET(2), Q at the "Press Start" screen.