This is for your GROUP not getting hit by the bloody geysers that pop up UNDER your feet NOT the ones that he releases in several directions.
Everyone pick a corner of a room and stay there. Attack the boss from range. He will come to one of you, which is ok. Just go around in a circle and get back to your corner.
The minute he summons the aoe that he shoots out in all directions, make sure you avoid that otherwise it'll slow you down for the next part.
Almost immediately afterward, he'll raise his arm in the arm. SOMEONE CALL out to start running in a clockwise direction. LOOK AT YOUR HEALTH BAR. If it goes down, you got hit even if you didn't. There's a random chance this'll happen. If everyone did not get hit, burn down the boss before he can do this again and pop goes your achievement. If it didn't work, exit out and reset the dungeon. If someone got hit, wipe, and you can try again right there. No need to leave.
The main problem with this achievement is that everyone didn't get hit when in fact someone did by the random chance and they weren't watching their health bar. Call it out if you see someone's bar go down. It'll save you time.