Serperior is kinda like CM Reuniclus in that it's another thing that makes stall teams hard to use. Most of its counters are offensive like shandera, blaziken, dragons. Most stall teams can't handle this at all unless they run bulky heatran and even then it gets subseeded if it doesn't have roar or at least resttalk.
EDIT: I think for a bulky variant, you should invest in defenses rather than HP as the less HP you have, the easier it is to subseed.
EDIT: I think for a bulky variant, you should invest in defenses rather than HP as the less HP you have, the easier it is to subseed.