What I did in the end to win was

Reaper crest, because we’re going to need a lot of silk, the range is helpful, and it’s pogo is less likely to send you into lava

Thread Storm for the skill, we are going to be using this a LOT

Subweapons are inconsequential, we don’t use them for this fight

The keys are the charms: the lava bell and, importantly, the barbed bracelet

Here's the theory: the barbed bracelet is not generally very good. But in this case, the boss has a very simple attack pattern where it is entirely feasible to just Do It Perfect. But there are additional complicating factors that get worse over time. So you want to finish the fight FAST.

The longer the fight lasts, the more likely you are to wind up in a horseshit checkmate situation and die.

So, put on the bracelet, do the fight perfectly. Probably die REAL FAST a couple times because getting hit for four masks HURTS.

Whenever the boss starts summoning, immediately jump up to where the add comes in and thread storm it. This will either kill it or leave it with one hit's worth of life, depending on how close you were to it.

THis is always safe as far as I can tell.

If he summons two, hit the one closest to him first with the thread storm and then try to immediately dash and jump accross to also kill the second one. Good luck.

You shouls also Thread Storm the boss after it ground pounds next to you.