It’s best to use small form to dodge the whirlwinds too. You can also use Lock on the bumper to switch to small form; you don’t have to press Y as described in the tutorial!
I really like that playing the bosses on Simple doesn’t reveal all their forms and the finale, so when you finally brave Regular you still get a nice surprise with new forms, attacks and visuals to wrap your (cup)head around and feast your eyes on. It would have been so easy for them to just have all the boss forms and their health/attacks reduced but this way encourages you to up your game for the real reward.
Unlike Rayman Origins and Rayman Legends where your dead bubblized co-op buddy floats around on the screen to be resurrected, in Cuphead your partner’s spirit drifts upwards and off the screen. Once they’re gone, they’re gone for the duration of the level which adds another layer of difficulty trying to keep the two of you alive with quick parries amidst all the madness. That said, if one of you does die, that person can then sit back and enjoy the visuals and sound a bit more whilst spurring the lone survivor on. It’s nice to carry or be carried over the finish line!