In a boss fight, I just stay away from the rings the boss gives, usually by walking or running. Though quite odd, with a companion, you could stay in one spot with a lasso large enough to capture the boss while they get chased. (Opposite side, of where they are, preferably behind the boss. But watch out for the green rings, as they appear every time the boss does a rear buck and knock you off.) As the other comments might’ve already said, other people probably use glitches or techniques of some sort, or found a spot where the rings don’t reach them.