I wanted to try a slightly different team. I dressed my Usha like a glass cannon and had Torrin buff her hp and defense so that she could survive. Well, this is making the boss recognize Usha as the tank instead of the tank and killing her with the Harpy's ultimate skill which is a single target skill that's supposed to land on tank. How does Harpy choose a tank? As far as I know whoever walks up to her first. She doesn't have any details of "attack high hp target" like vortex or anything like that. Even if she did, after the Torrin buffs, Usha still has less hp and defense than the tank leaving me thinking that the only thing triggering this issue is the Torrin buff itself.
I can play Usha without Torrin and she does not become the tank.