Given just how extensively these two fights can absolutely fuck you up, preparation is a number one priority, because while there's leeway in most boss fights to figure out how to fight your own way and still win, this is a very different story.
Even more than Radagon and the Elden Beast in the base game, Shadow of the Erdtree's final challenge is the sum total of all lessons learned, because make no mistake, you will need to employ all of it to survive.
High Vigor is nice, but you're gonna need as much raw defensive power as possible here. That's less about softening blows, more about narrowly avoiding getting one-shotted if you take a hit.
If you fully completed their quests, and they aren't, uh, downstairs from the boss room, Thollier and Ansbach will be available as NPC summons for this fight.
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