Unfortunately Bullseye was a terrible choice here, especially considering that there is a bleed immunity node on the fight too.
Your best bet is to grind through it by hitting Sassy in the block only. Thankfully Bullseye can crit through the block so you don't lose all your damage, but I believe that hitting through the block should prevent his regen from triggering.
Incoming record for longest fight?
It'll be up there, but moreso revive intensive than time. With the protection node, you've got no more than a minute of dealing any damage whatsoever before he reduces all of it. So you're kinda just hoping to get the last 500k-ish health down in 10 revives, though I would guess that it will take more. Depending on skill level, it could be a significantly better option than restarting given where Sasquatch is in the quest.