Aria can use her Reversed Words of Nurturing to destroy it over time. As for Rita...well, I've held back my Extended Haste this whole fight for a reason. I have a few tricks up my sleeve, none of which explicitly induce its destruction, but I'm running out of spells anyway. There's always the wand...and Polymorph, but I'm loathe to be anywhere near something that could eat me...unless I turned into something huge inside its stomach and killed it from the inside...