I think they have about 150 HP. What Party do you use? Peach should be in the party for healing purpose. Her Group Hug is awesome. Just use Bowser's timed physical attacks and Mario's normal "Jump" until they go down. Focus at one of them at a time. When there's only 1 left, it will escape. Don't waste Bad Mushrooms in these battles... you'll need them for Jonathan Jones.
Also, Smilax is BY FAR the toughest boss so far. He makes Belome 2 look like a patetic wimp.