It should be a bit over 6000, eyeballing the stat topic figures and putting Comet2's multiplier in their place.
Honestly, if Cait isn't allowed to remove Shield, he loses for sure. (Shield cuts HP by 10%, bringing Cait down to within a hair of average.) Without that, Cait has 111% PCHP. Base Dark Wave does 132%... against Level 40 PCHP, which is a bit overlevelled (but on the other hand, stat boosters and masters and so on muck with things). I'd kneejerk Comet2 taking out around 1/6 of Fou's HP, which is practically a survival roll, and a few interps could send this either way. Really good fight if Shield can be removed (or if it, or Shell, reduces Dark Wave). Not sure what I make of this personally.