Aint got time to bleed is for killing a boss(Bounty) with one block of health left. I did this on Mission 1 The Hoodlum Dolls Stage 1(Sweet Angel)
as he is the easiest boss in the game.
Once you get to the Boss fight keep shooting Sweet Angel until you only have to shoot him a couple times to kill him , now let him damage you or shoot you until you have 1 block of health left. Once you get to 1 block of health left finish him off to unlock a very easy achievement.