I'm going to come right out and say it - this is STUPID! Old Scratch only promotes if you successfully complete this dungeon, and there is NO WAY to get help with this because it's not on the team-up, either at the sigil or the ledger.
I tried several times and used up my remaining crowns and still failed.

Now my Witchdoctor will HAVE to farm for Obsidian keys and will HAVE to buy henchmen, just to promote Old Scratch.