Root Abyss Boss Mechanics

I take half the time on Vellum now after noticing something. At the beginning of the fight, he'll only pop out of the ground and quickly go back in. Idk the exact % but around 80-90% hp left he'll start using the 1hko fireball attack. Once he does that he's a punching bag as he tends to throw out 2-3 fireballs and you can just stand under him or behind him. Until that point it's best to just use heavy burst damage as soon as he pops up (For my AB that means not using soul seeker (long windup time) but rather trinity/roar until his fireball attack comes up).

Anyone know exactly what his poison thing does? I just shrug it off and tank it but it may be a problem during chaos vellum.