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