yeah melodic minor sucks. don't use it in metal. it's just a major scale with a minor third and it really only sounds good in jazz or other genres that you don't want to listen to. the only way i could think about using it in metal is if you are playing in a major key and you want to hint at a minor scale for a bit. but that's really just borrowing from the parallel minor.
forget what you know about Melodic minor and go back to good ole' Harmonic minor! you'll find metal all over that scale.
i've found that a lot of metalheads will switch between harmonic and natural minor. it's easy to do if you know where the root is. all you have to do is play natural minor and then throw in the leading tone (major 7th - one fret below the tonic) every once in a while when you want some dissonance or you want to resolve to the tonic. also, by doing this you can use diminished chords as much as you want and it sounds evil. \m/
it's easier to look at the scales in this order: