Red crystal

the final is quite easy, even compared to a battle with a random guest – usually, all guests are stronger. I defeated all the guests, and they stopped appearing; I cleared the entire map, only leaving the titans near the maxer's amulet untouched, as I didn't want to go back. They didn't give me level 34->35, so I only reached level 34 and then used trees to level up to 39, after which I completed the ghost quest.

This took over 9 months on a single hero.
And if you judge by the guests, each one had an average difficulty level (only low at the moment of arrival, before the guest levels up), and it was genuinely average; you can't use auto-battle, and you have to manually replay some battles several times.