For those wondering, we didn't die mostly due to a well-placed Disintegrate. My "stick an immovable rod in its innards" was disappointingly low damage.
For those wondering otherwise, our best plan involved turning it into a regular worm, using an immovable rod with a hole drilled through the length of the staff, clicking the rod, then dispelling polymorph.
Who forgot to poison the monk beforehand?
Given that they're immune, you may as well always keep them poisoned, just in case.