When Dr. Zomboss comes down to throw an ice ball or fire ball, you can use Freeze Shrooms to freeze him. This allows your various pults time to get some damage on Dr. Zomboss himself. Jalapenos will also cause damage to him when he is down to toss a fire or ice ball. The more damage you can do to 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.