thanks for your answer. no the jump doesn't kill me the lack of healing does and of course those little jumps gather one by one does too.
If it's little jumps adding up, then it sounds like you are attacking Toth, not Zorn. This must be healed through.
There are other possibilities. If the bosses are 30 meters away, and you're standing behind Toth, you may be within 25 meters of Zorn and getting fearful anyway, even though you are on Toth. I don't believe you should ever get Mental Anguish on the Toth side. There is also frontal cone damage from Toth you may be eating if you're in front of him. \
Normally the melee attacking Toth doesn't get any heals from me until after the first tank-swap. By then I expect a melee to have a little over half health. If you're a melee heavy group, it's helpful to have a sorcerer who has good AOE heals. Revivification effectively cancels the damage of Toths' little jumps.
If you mean during the "sonic" phase with lots of little red circles on the ground, there are ticks of damage raid-wide from Toth pounding the ground, that damage is unavoidable. I forget if it's there on Story but it's definitely there on Hard and it will eat up most of your health even if you avoid all the red circles.