This achievement is one of the trickier in the game, but not hard as long as you are paying attention. You CAN reload your previous checkpoint if you get hit. I recommend doing it on the very first level, but not on Easy (as it is difficult to judge your life bar on Easy sometimes since damage is so absurdly low). For best results set the speed low, to 75%, to help you keep better track of things. It is also important to note your armor bar as taking damage as this does count as a hit. Either keep your armor bar empty or full for ease of visuals (use the Goodie Fountain at the start of the level to go ahead and fill it up). Otherwise simply play through the level, taking it slow and reloading last save if you get hit. You can check if you did or did not get hit, by attempting to pick up health/armor (the Goodie Fountain comes in handy here if there is none in the immediate area).