After you unlock infinite ammunition with the RPG, go to any level. Run up to any wall and shoot at it close-blank range with the RPG. This should bring you into "dying status", and Sheva will try to o revive you. Tap Circle just when she does and Chris will say something similar to "Thanks" or "Thanks for the help". Continue to do this to thank Sheva and you will unlock the achievement or trophy in about five minutes as opposed to having to play through the entire game.