Is this a bug? I got this achievement recently, but it says that you have to be playing on Seasoned Difficulty or higher, and I've never raised my challenge difficulty beyond the standard Adventurer Difficulty. The only thing that I can think of that could have triggered it is that it popped when I was doing a daily Undaunted quest and was fighting a delve boss as part of it and there was another player fighting him as well. Could it have registered that we were "together" even though we weren't grouped, and they were playing on the higher difficulty, and somehow I got credit for it, because I just happened to also meet the required number of defeating 30 bosses even if I wasn't on the required difficulty?