How to remove stuck locknut from porcelain toilet?
If the plastic part you are removing is headed to the scrap/trash/recycle it might be ripe for &quot;crush it with locking pliers&quot; as a brute force removal technique that should still be low-stress for the porcelain.
At some intermediate level, grab it with locking pliers to unscrew since it does not have much for a wrench to grab might apply, but it might just break anyway.
If you are trying to preserve it, but remove it, good luck.