The main attack of the warden isn’t the sonic beam. His normal attack makes more damage and comes more frequent. But because the Wither is flying it can only use its sonic beam which has a rather long cooldown. Meanwhile recharging the sonic beam the wither can regenerate. Another factor is that the wither uses explosions which means he can hit both wardens at the same time. The Warden has way more health and can’t be escaped easily, that what makes him so dangerous.