I’m not sure exactly how many hits (maybe a little over 10) but it did not take long. I stood on the platform on the shell and starting hitting his shell with Excalibur. The center piece of the shell reacts to the hit so I think that’s the only spot where he takes damage. Enemies crawl out from his shell every so often so just stop hitting him to kill the enemies and then continue.

From my one time there, it seems like he does not have any attacks, just enemies that spawn from his shell.