Just to add some info since it seems nobody is aware after all this time, there is a way to handle the mechanics where you can remove the damage shield from the summoned wisps.

That is what I was referring to. =P

Oh! Thanks for the hint Krin!

I've still got no idea how to do it lol but it's nice to know it's a possibility. I did manage to brute force my way through with a water/air/fire, but only barely (though tbf my air is barely rank 2).

okay, so the shield vanishes when pyro goes down, you just gotta,,, take down pyro without hitting the wisps at all, kinda hard with fairy light and razor wind both being multi-hit moves at r5 but doable.
and I have to work out burning embers debuff (is that pyro full mana move? Why does thorncoat not block it from being inflicted if it is when thorncoat can block basically any other debuff like that)

In summary:

Attack Pyro

Block at full mana

Avoid hitting adds when summoned, attack Pyro until magic shield is down from adds,

Defeat adds before bomb timer drops,

rinse and repeat

The plot twist I wasn't expecting was that it seems Pyro pulls out the move that inflicts burning embers to be about turn 6 of the fight usually? (It could be always turn 6 and my tank just managing to dodge the move sometimes, or it maybe it's based on damage done? But the times when I've remembered to pay attention it often seemed to be about turn 6 it was inflicted anyway)

Thank freaking goodness I found this...Ive been struggling with the pyromancer for months now and was seriously getting frustrated. I do run a double fire team so yay lol This time I was able to beat floor two pyro /and/ did it without using any draughts or philters! I did put a greater ent flask of mitigation on all team members(I found that one is the most helpful) and of course training grounds buffs

For my fire peeps, use attack 4 and 5 within the first few turns to get health down as much as possible before they summon the wisps. Once the wisps were up I only used quick attack and attack 3 and the pyro was down before the wisp bomb went off :)
My water tank used attack 4 and then 5 to increase duration of ice sphere as much as possible. Attack 3 was used once it had mana back to help with the fact that quick attack does piddly damage haha