Damn. =( I guess I'll just have to raise the vitality stat then, as my skill is already at 50 and I have 20.3%+18%+18% physical attack gems. What buffs would offer the best increase for visceral strikes against Kos?
You should at least farm the Winter Lanterns right before Kos for some cursed 22% gems. Against OoK any "ATK down against beasts", "up stamina cost" or "weapon durability down" should be ok. KoS is only regarded as kin in his second phase apparently. Generally for cursed gems the only ones you want to avoid are "ATK down against beasts", "ATK down" (as it often negates the benefit) and "HP depletes" since there are lots of beasts in the game and not that many kin. OoK is one of the few bosses where the "ATK down against beasts" is fine.
So equip all the eye runes you've got, the best frenzy resistance armor you have and stack up on those Bold hunter's marks. If you do it right you can kill both Winter Lanterns in the caves while only taking one frenzy. It's generally cheaper to spend a few blood vials rather than use sedatives when going against them.
My tactic is to use LHB 2H mode to kill them, take any item they drop and quickly rush to the second one and kill it then spend two vials to get to full health so even if the second frenzy hits you won't die.