White Tyrant's Challenge is more time efficient than Hidden. Play it on hard and you'll finish it pretty quickly. I'm not sure how much it'll take at lower levels but after 61 I'm doing it in about a minute and getting over 70K EXP. Hidden on the other hand takes at least 3-4 and only gives something like 90-100K. Factoring the time between boats, waiting for the stage to end, etc does not even this out. It's also easier than Hidden, imo. Especially being that you're a staff Evie and don't have to worry about him hopping around the map.
Quote from Overblood;644555:
Two moons
I hear Unfinished Business is pretty good too, but the boss hits hard as hell. Be ready for that. Two Moons is definitely easier. I'll need to do some more testing to figure out which is the most time efficient overall, but it's not like one is far better than the other as far as I can tell.