This achievement is not real difficult and there is way to help yourself get this one. Boss doesn't matter, but I'd suggest the following weapon choice:
The blast radius/area of effect on it means you could hurt yourself. While fighting a boss (I went with the first one on the Foyer) whittle down the bosses health while getting yours lower (around a 1/3 or less). As soon as you defeat the boss, aim straight down and keep firing until you die. Hopefully you don't have much health left and can kill yourself during the explosion.
If you don't get it, just try again. Took me a couple tries to get it but it wasn't entirely too difficult.