how immune is immune (boss spoiler)

In the wintertide theres a boss called Dalvo's Astral Projection who is immune to Crushing, Slashing, Piercing, Fire, Cold, Poison, Nature, Acid, and Trauma. Which is all my attacks as an Archer/BC. If I have a mod say that puts a fire dot when I use a bomb attack is he also immune to that since he is immune to fire or are mods treated differently? Should I need to skill up 2 alternate skills just for that one boss? Currently working mentalism/priest, but its going pretty slow so Im asking to see if its really necessary or if theres another way.

If immune to Fire, it will be immune to Fire DoTs as well.

Note that immunity = 100% resistance, so if you lower resistance the boss is no longer immune. For instance, lower resistance by 20% and it becomes only 80% resistant (your damage is divided by 5).

All you can do is return to that boss prepared, with resistance lowering gear, or with attacks that do damage of other types, like Darkness or Psychic.

He's not hard to do as long as someone in the group can damage him. You can focus on the other mobs, or healing/buffing. And others in the group might be able to lower his resistance. A warden for instance can make him take 20% damage from fire/electric.