Rin, as in the succubus?
I beat her at level 6, CMIIW, she got stronger as you got stronger, so no grind.
Beating her is all about luck and strategy.
I use the magic skill that fills up TP bar completely, and use a skill in Fire Stone that restore mana equivalent to the amount of TP consumed (buff, doesn't end the turn), then use that 1 skill that use all of your TP, and at the same time filled up your mana to full again (because of the buff). So I've unlimited mana and TP for about 3-4 turns (until the buff expired), back and forth.
But like you said, sometimes she deals massive damage, sometimes she just use normal attack.
Took me around 2-3 tries to beat her, though.