I think you should go with:
Fire: Either Darmanitan or Chandelure
Water: Seismitoad or Samurott
Electric : Eelektross (Due to its amazing ability)
Try getting Zoroark (Cause it can easily solo the elite four)
Train these Four to around lv 50 evolve elektross at 49 and I guess you'll need two HM slaves .
For hm-slaves try : Panpour and Golurk .
Comment if you need move sets.