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.
We had a dm that got tired of extensive battle plans of over an hour (fair) so would start a timer when a new encounter would pop up or we started taking a huge amount of time on something. Every 10 min they would drop a d20 with a "random" thing that would occur to the party. Positive or negative things can occur. We got the picture quick. It was fair, we liked to plan almost as much as fighting lol.