To be honest, from the previous chapters when they were fighting the tyrant. The tyrant did used two AOE attacks, the hand slam shockwave and the voice attack BUT the reason why he was safe was because someone from the vanguard was in front of him like misaki when she was not in the party and still did not take the damage because she was next to a vanguard. In this case, no one was next to him to take the hit for him.
They don't need a tank, they need a vanguard to be in front of him and shield him from damage.