I think you got the bosses mixed up. Toth is the one that jumps and Zorn gives you the fearful, attacking Toth does not reflect any damage.
Ah yes. I'll go back and fix that.
Alternatively do not wait for Toth's jump, keep an eye on Zorn, he plays a shriek animation, all DoTs get cleansed and you get a debuff, this is the time to stop attacking, moments later Toth will jump.
OK, I can try that. Are you sure the two are tied? Eventually (after about the 10th wipe last night) I got into a rhythm where I could predict very roughly when the jump was imminent, but that was entirely based on the DPS others were putting out. With a different group the timing would probably not be the same.
Still, the solution I eventually hit on was just not attacking with anything other than force breach, and slow time after the first half of the DPS cycle on him, for safety's sake.