This is one annoying battle but I just finished it without having the Shedder polymorph into the 70,000 or so HP form. We used Hydra and it takes away three of his hit points, so if you use Hydra (or Chimera I assume) then the Shedder will die and you will only be left with his minions to worry about. It worked quite well so I hope this helps!
Shoutout to Malvin Jadewalker for using the Hydra and to Blaze Firefriend for helping us along with it!
For those wondering if this actually works, it does. As of now, i hit him with a hydra when he had 2HP left and it straight up killed him. Just wanted to confirm this for others trying to do this boss!
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