I think people over-exaggerate how "Easy" they find Sophia. There is a lot that can go wrong when you're fighting her, and I know that I got pretty down with failure, after failure, after failure, after failure, after...you get the idea.

Like you, Sophia's the first time I'm trying to tank something that's brand-new end game. And I also had a lot of trouble getting one-shoted by her tankbuster for a while. Still do every now and again. But as long as your co-tank and heals are on-point, it gets much easier as you pick up on the pattern of its casts. Like she'll usually use it after her triple-hitting raid-wide AoE, I forget what it's called, so it's a good idea to pop your CDs right around the third cast so that there's no time conflict or error.

All that said...I do think that Sophia is easier than the previous couple of trials, Nidhogg and Sephirot, both of which I feel were a lot less forgiving with regards to their mechanics.