What about a Spell Trigger full of Lower Resistances with a sequencer of Great Malison and, maybe, 2 Flame arrows. Then a single FoD finishes the beast

Or you may pack up your CC with ADHWs, or FoDs. Or use all spell-storing things to cast a rain of magic missiles on the beast
Pelting him to death with Melf's Minute Meteors, regripping on your SotM after each attack so staying invis should also be funny, but cheesy