I've heard somewhere that most if not all the Misfits songs are played downpicking ( correct me if Im wrong) and Im learning their songs but I cant downpick fast enough for their song cleanly and tightly so can any of you guys be helpful to improve my technique?

Yeah, just practice the songs a bit slower until you find a pace where you can do them all downpicking then bring the speed up. It doesn't actually matter massively, Doyle always downpicked. You just need to hit the strings hard.
I don't think all misfits songs are played downpicking!
Listening to songs like Helena, or Dig Up Her Bones, then it'd be nigh impossible. Just play it so it sounds right!