Most wipes I see on hydra come from repositioning after hydra does the move in the center of the room. The group just scatters and the tank pulls hydra back to a wall and people cross paths there causing the fight to cause issues with fear etc.

What we did is we designated a specific point for the tank to take the boss back to (The Open Gate) and told the group to stfa from it. And most importantly, not to cross that line. So basically we said imagine a pie missing a wedge (the missing wedge is the tanks domain to return the boss to the open gate) no one is to touch or go into that wedge, go around on the outside of the circle in the opposite direction. IN this way people stopped dieing and we won.