Please be more careful that the information you contribute to the bug forum is correct. As pointed out in the sticky, staff wastes a lot of time on poorly researched and incorrect information.
Captain isn't immune to anything. He's highly resistant to cold and magic.
[Fri Feb 28 21:28:35 2020] A Kromzek Captain is consumed by the flames of the sun. (fire)
[Sun Sep 13 14:35:10 2020] A Kromzek Captain has been poisoned. (poison)
[Wed Sep 30 16:20:30 2020] A Kromzek Captain's skin freezes. (cold)
[Sun Sep 13 14:34:41 2020] A Kromzek Captain's skin erupts in purulent pock marks. (disease)
[Tue Mar 10 02:24:27 2020] A Kromzek Captain feels dispelled. (unresistable)
And dictate lands as already mentioned. (magic)
As far as I can find, this is consistent with live since there are Luclin era posts that reference difficulty but success slowing him with tash (possible they meant disease slowed but they did mention tash, might be worth testing dictate or theft of thought on a live server), e.g.
https://everquest.allakhazam.com/db/...02630463086872
Interestingly, using the giants from the quest to kill WW dragons was done in-era. They had to add a respawn timer to the quest (or fix some other issue?) because people were spawning multiple waves to throw at Sontalak.
https://web.archive.org/web/20021107...=1255&start=75
Seems like confirmed classic to me.