I have been trying and trying and it seems that what makes this a hard thing to do is because there are two bosses. I have tried this with human assassin (to maximize damage) and have been using some mid-battle leveling. I beat the other boss (a lvl 10 wraith) at level 8 (or was it 7?) and the problem was running out of other high level enemies (i was left with like 5 level 1s and a level 9+ frank.
Since everyone was immune to poison, i just converted my starting glyph to damage and my final damage (at level 9) was 85. Frank had 650+ hp and I already ran out of MPs and pots and I only had like 3 HP potions left (which wasn't enough).
Is there something I'm missing with mid-level battling? Is it really just too hard to beat with an assassin and will require some epic luck?
I have also tried polymorphing all the enemies (jehora) at one point and the wraith was transformed into a golem but frank stayed the same.