I found a tactic for The Harlot. First of all, you need a lot of mana so you can stray through the battle. She uses Dragon Eye every 3-4 turns (not sure, haven't counted properly) so you can predict her actions pretty easily. Meaning that putting up a debuff doesn't hurt. The only thing that sucks is her abuse of Taunt in the second phase, luckily she's almost dead at that point. Second, you need a phys resist magatama. Her focused attack will always hit the Demi-Fiend meaning that he will suffer the most. You'd think that an electric magatama would help more, but for me, a phys resist magatama worked better. Debilitate is a really nice skill to have at this point, though you'd need to make a good Amaterasu to have that.
Another key factor is having really strong magic. Magma Axis works fine, but I used all ice attacks with all of them being boosted by Ice Boost/Magic Boost just so I could get in some sweet physical damage with Deathbound or Divine Shot every now and then. A few helpful demons that I found were Odin and Wu Kong, though I didn't have Wu Kong since I fused him away. The last phase can be a pain, but it almost seems like Dekunda is completely removed from her skillset whenever she uses Dragon Eye. So you can even predict that pattern.
I'll admit. It took me a lot of attempts before I could do her. Really difficult boss, but really predictable after you've failed a few times.