One of the later achievements was missable in a way that I found a bit unfair. So, if you beat the last normal boss, a new encounter becomes available that gives you a chance to take on a nastier version of the boss fight and get one of the trickier achievements without having to know ahead of time what to set up. I feel like this is fair to all players and doesn't reduce the difficulty of the encounter in the least. Good luck everyone! :)
Small Patch
