Instead of nerfing this fight. Simply just cut this fight out of the quests and turn it into a raid. It already has raid level mechanics, would just need a damage/hp boosts to match. Maybe the damage from the mushrooms fine tuned a bit though for higher reaper pushing. For the gear, just take all the "new" gear and make "attuned" versions of it with raid dice and ToEE set, if we're just looking at the previous items being scaled up with changes done to current standards.

Would really hate to see this fight toned down and lose it's uniqueness. One of the best end fights in the game.
As I always called it. The best non raid, raid boss!