Conspiracy nuts gonna conspire but to the best of my knowledge Drissa was not very imm bonused in direct ways... Lightforge edge, leader weapon were really it, I was grateful for vanguard but it's not like it helped land any kills. Edge points for imm xp are minimal, getting to 3k gave me enough for keys to success.
I have it in my head that lots of imm XP and/or lots of hours might give undocumented perks, just based on my own encounters with other peoples characters.
Also can't overlook that a lot of people don't spam skills so you get 50 hrs of domination against them until important stuff like enhanced reactions, riposte, whatever catch up. Not sure if this has anything to do with shifters.
I also frequently had 40-50 hitroll to go with a high damroll, I feel like it might help in also undocumented ways despite what common knowledge says. This is totally anecdotal but I always highly prioritized 25 hitroll unbuffed.
Finally swiftblade edge is really very good and all warrior elfs should take it. When combined with concealed/reckoning you have a boosted chance to return damage on any dodge or parry. Flow helped return even more damage. I wondered if swiftblade might increase parry chance somehow too, but have no data nor enough log parsing knowledge to check, and there are lots of other variables. Nerfing people's str to 3 helped parry against them, I'm sure.