The HA hunter I use is a Gallade with the following moveset.
- Leaf Blade
- Thunder Wave
- Skill Swap
- False Swipe
The most important moves on the set are Thunder Wave and Skill Swap. Thunder Wave for the paralysis of course, and when you end up with the Pokemon with the desired ability you can paralyze it so that it cannot call for anymore help. Skill Swap is for checking the ability, and it will only fail if the opponent uses a protect move or something to prevent non-damaging moves. Even if you have the same ability it will work.
False Swipe is mostly for weakening the target, but isn't mandatory. It's of no use on Ghost types of course. Leaf Blade is there for KOing everything else, though any move that nothing is immune to works perfect (so no Normal, Fighting, Psychic, Ground, Ghost, Poison, Electric, or Dragon type moves).
I always let my Gallade hold Leftovers. You won't need to heal very often so long as it holds it. I recommend having lots of Leppa berries to restore your PP, as well as stocking up on potions and Full Heals in case you get poisoned or burned.