finally beat him. just raw luck/beatdown.
I did go further, and got a scroll of legendary proportions, then went back which helped alot.
icy prison/polymorph totally failed. What ended up working was just attacks/round, hitting on crits. and notably pentas thundercall. thundercall dealing 1d8 sonic, and having a fort save for stunned. it skips spell resist. so just straight fort save.. now, he DOES have like 29 fort save, so, only failed on a 1. but repeat every round, and it gets through once in a while. which forces him to skip a turn, and removes his dex AC. generally able to get some dps in when it happens.
Also, the sendri question. icyprison/poly morph every turn? (poly when freedom of movement is up, prison when its not)? or, dispel every turn to try and force him to rebuff, or get some of those AC buffs off? as well as divine power/greater magic fang?, or, have him snowball/mm/battering blast every turn maximized/bolstered? again, poly/prison attempts all failed. dispel every round was not going great. his spell resist is so high, spells often fail outright, never mind the spell DC after the resist.
so, again, the raw snowball, bb worked. just paired with hits/round.
I think penta's thundercall proced twice on the win. which is huge luck. also seems to be bugged? was only doing 1d8. level 11 I think its supposed to do 3d8.
mark of justice/home guard would make this fight cake.
older version of the dlc I think had issues with the silver regen. all the guides talk about buying a mithral kama much earlier in the game so you have a means to kill him. idk, but currently theres a shopekeeper in the area that sells mithril, and alch silver weapons. also, sendri can get silvery barbs spell.