Bloody MessG 20

This is a very rare achievement but is quite easy to get. You want to do this solo, because if anyone gets hit I believe it voids the achievement. This is the first boss in the dungeon and only takes a minute to get to it, it can be done on normal as well, no need for veteran. I found Sorcerer to be best to go for this because I could damage the boss from range and DPS it down quickly and with boundless storm it boosts your speed for a few seconds making it quite easy to avoid the Bloody Geysers. It might take a few tries, but with a good build you should have around 70-100K+ DPS and if I remember correctly the boss only has around 800-900k health on normal, so really you should be able to easily kill in 10-20 seconds and if you avoided the Geysers properly it will unlock. If it doesn't just exit and reset and try again.