Hmm.
One of the things that always annoys me in Flash is the shortcuts many people take, even the best animators. However, while I can't be sure, it looks like the vast majority of this was done frame by frame, by far the best way to animate. It pays off, and the animation is both smooth and enjoyable to look at. There's no awkward movements of characters, and the syncing is done well. Not to mention, you had me loving the werewolf (I dunno what it is about him), as well as liking the zombie's "lead singer." Impressively done.