Tornado bird boss the tornado doesn't dissipate after casting

Not sure if this is intentional but how is a melee build going to beat a mechanic that doesn't dissipate. The boss goes into his tornado move, creating the huge tornado surrounding himself, and several small tornados around him. At this time it effectively makes him immune to damage as well. Well I dodge it and dodge the big tornado, and he just is able to stay in it and attack, i can't get close to it, and wondering if this is a bug. Seems like entirely impossible fight if he gets this move off before I kill him.