The Down B in general is super obvious. Just interrupt or dodge. Just straight up blocking it is silly. It's like trying to block a up tilt from Ganon. You just don't do it. Better yet, play as a character with counters. Like Lucina for instance. Her counter is based on how strong the hit is. If Bowser is as strong as you say then, the counters would just massacre him. I myself never had real trouble with Bowser players. They are easy to figure out. But I can't really talk since I wasn't there to see your experiences for myself. Also I find this iteration of him to be the best version of him yet. I like him just the way he is and I don't even play him.