Red crystal

the final battle is actually quite easy, even compared to a random guest encounter – usually, all guests are stronger. In my case, I defeated all the guests, and they stopped appearing. I cleared the entire map, only leaving the titans near the maxer's shrine untouched, as I didn't want to go back. They were the ones preventing me from reaching level 34->35, so I only reached level 34 and then used trees to level up to 39, after which I defeated the ghosts.

This took me over 9 months on a single hero.
And judging 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 rely on auto-battle, and you have to manually outplay some of them several times.