I just finished the game a bit ago, but I'm having trouble remembering. I think you're talking about Queen Bean so here's what I remember doing.
First, jump on her arms until they become little wimpy things. Of course, make sure to dodge all her attacks, like the shockwave and the bean spit...I think there's another attack but I don't remember. Now, once both her arms are gone, her crown will pop off. That's when you pull out the Bros. attacks, any one that ends with jumping on her head or whatever. Eventually she'll get her arms and crown back, but just keep repeating. I think that's how it went. Good luck!