I've soloed (A) Yorcia as BST a number of times using a tier 1 torso. As long as you know which map you're on, where the mobs are, and are able to kill quickly, it's usually do-able. It's mostly a game of balancing time spent killing for puissance, and time spent getting back to the book to summon valiants.
Oh, and I have to tip my virtual hat to Josi, since it was his video that gave me the know-how.