To breed the Zombie Dragon you will need the meteor and pollen dragons. In its description it says this type is a rare and misunderstood dragon, is a reliable crowd pleaser. Just be sure to heed the wizards stern warning to only feed it with dragon treats. Anyways, always remember that breeding left or right and the level of your dragon does not matter. However, level 15 and above gives you much better chance to succeed.
The process may take up to 20 hours or if you have the money, you can use 20 gems on it to speed up the breeding. But I'm guessing you already know that so yeah, just remember that level 15 plus dragons will give you higher rate of success. Again, the combinations for
breeding zombie dragon in Dragonvale are Meteor and Pollen.