To get this achievement you beat one boss with the last bit of health you have. it can be done in any mode, but I personally don't recommend this for Hardcore mode, but if want to get it in that mode good luck since bosses do more damage. probably the easiest and earliest attempt you can do this is playing Main Game mode and facing off against the Tinkerslug (P.O.O.P T.O.O.T). What I did was i got hit early in the level and by the time I got to fight each I purposely jumped into the attacks and the boss itself at certain intervals, and by the end of the fight only needing the last few hits on it I was at a quarter of a heart (you start the game with two hearts and everytime you get hit you either lose half a heart or a quarter of a heart depending on which mode you choose). Once the fight is over the achievement will pop up during the cut scene.