When Dr. Zomboss comes down to throw an ice ball or fire ball, use Freeze Shrooms to freeze him. This gives your pults time to get some damage on Dr. Zomboss himself. Jalapenos also inflict damage to him when he is down to toss a fire or ice ball. The more damage you inflict on Dr. Zomboss, the faster and easier it is to end this level. Save at least one Freeze Shroom and one Jalapeno for whatever he tosses.