Equip the lowest damage scythe you can find, go to Castle Zvahl in the past, and whack on a fortalice until you deplete its 5000 HP. By then the other fortalices should have respawned, since they do so after 30 minutes. They don't fight back, and as long as you have sneak/invis you can move between them easily. They are level 75, so you should be able to cap skills for level 82 or so.