UnderMine

This achievement requires you to unlock The Crone Valeen and the ability to Hex yourself. You do this by completing the entire game one time. Once you do she’ll appear on the Hub standing next to Kurtz. Apply the Rogue’s Ultimatum hex and you’re good to go.

Strategy wise, this will work for all five achievements based around killing bosses with this hex. It’s entirely focused on the Resurrection relic. This hex returns your character to square one and removes all your upgrades. It is brutally hard to even get to a boss let alone kill it. What you want to do, is play until you find the Resurrection recipie. (Ideally, you DONT WANT to craft it at the blacksmith until you’re ready to go. If you already have that’s fine, it’s just harder).

Now start your hex run. Your goal is to find Resurrection ASAP. If you can craft it in the beginning great, if not you’ll have to find it in the mines. Once you find it, you basically become fail proof.

With Resurrection in hand, once you die, you will be brought back to life at the expense of the relic. Once this happens, open the menu and quit the game. Since the game only saves at the beginning of each floor, when you come back you’ll be at the start of the floor you died on, all relics intact, including resurrection. Then simply try again, quitting every time you pop it. Learn the floors and the best way to get through them, and before you know it you’ll be able to get through the bosses.

It takes a lot of time and trial and error, but with resurrection you literally can’t fail if you quit after dying once.

EDIT: I also recommend the familiar Sol Phoenix. His level three ability is a resurrection as well. With two rezzes and as stated below the summon stone set to 0, it becomes way way easier. I also recommend saving a float potion or finding float boots for the hydra boss. Makes that one a laugher