Hm. WHAM is changing its damage estimates throughout the fight, interesting.
The monster seems to have 75% damage resistance across the board (Ashes and Soot should be dealing 50% + 50% of your buffed myst = 720, but is only doing 180; spectral snapper should be doing 20% of buffed mus + 10-40 = 254 - 284, did 65 which is in range for 75% resistance). At least, those two data points seem to be consistent, but if you want to be more confident, I'll need more info, since I don't really understand how WHAM updates its damage estimates.
What weapon were you using? (wallop, kneebutt; also relevant is +weapon damage, e.g. from passives) And what pants did you have equipped? (kneebutt; also relevant is if you have Tao) How much +spell damage did you have? (for saucegeyser)