Easiest done on the first level. Be careful on battles and i recommend starting the game on easy to get this since enemies arent tough and they wont hurt you a lot if they succeed in hiting you. Make frequent use of the healthcanisters littered through the level and you should have any problems. Careful with the first boss though, he might prove difficult. Keep an eye out for the indicator for his special attack and avoid it as soon as you see it. Achievement unlocks after the cutscene where you free the prisoner.