Any tips for them? Tried Advent Bay(even tried to make a separate save with Level 99 max stat characters to see if it becomes easier) and it's murder. The super-high HP and Undress/Prevail(iirc some have the latter too) III makes my life a horror.

So any tips? Characters best used for it? Maybe a different map to try beforehand?

Advent Bay was the Slime one yes? Personally i think it's the hardest anyways, maybe just because it is kinda fake from a Himegari lvl, personally i found the Phema(and mirage Forest) ones a good start, goin with Phema Sluicegate, then Mirage Forest(can take a long time cuz of all that annoying cover) and finally Phema Mountains~

Get weps that the monsters are weak against and also get Undress+Prevail for the Heroines and also Mylen learns Prevail III
Also personally I found that dungeon to be the easiest, hardest being Valkyrie Falls (due to all of them having insta-death skills) and Phema Mountains (due to all the areas u need flight and grounded and the monsters all having Nightmare Lightning which keeps charming me)

From what i got out of some tests the insta-death is a darkness ability and was avoidable with X or higher resistance in darkness.