First time through, the most difficult part was the gatekeepers before Atheon, just getting everyone where they needed and when. after about an hour someone quit, we replaced him, finished the gatekeepers, now the issue was trying to physically dish out enough damage to kill Atheon before he enrages. we had to get creative. After about three hours we finally got our strategy and we worked it like clockwork and killed Atheon with a perfect kill essentially. (nobody died). The second time through, the hardest part was getting the vault to open. We all had our strategies in place and we didn't get wiped more than 2 times per fight. Everything worked like clockwork.