I like Aes the most as long as the other 3 people in my party aren't Kai/Chain Vella/Staff Evie. Then it just turns into a laserspin deathball spam rave party.
Arcana is a close second but her tracking multihit combos are trash and failing the torture minigame due to low dps sucks. Dullahan has the worst time wasters. Rupacitus somehow manages to attack over my max height Karok's head whenever I'm trying to get him with pillar counters and his flip attacks are really annoying to block. Also bubbles.
I think Spinning Slash and Deft Reload are best to prioritize having on accessible quickslots for their instant stamina restores and watching DR's cooldown. My Z skills are Transform, True Shot, and Desperado and it's pretty comfortable.
I thought this was always a feature of the black Elchulus raid. He loses the blue fire breath and the Godly Inferno debuff from it when his hp is low enough for him to do his tired animation. You can see it happening in this pre-ignition run at 44:58. It's been an easily missable detail since everyone started destroying S1 bosses.
I don't think this is a bug unless it was happening when he had more than 10%~ of his health left. It would look more natural if all of his breath attacks dropped the blue effect at that point in the raid. Not sure why they only applied it to the one attack, maybe because it's the most defensive looking one.