I don't know if I should slow down the opponent, or blow up if I can't take 'em down.
Also which would most likely be better in this moveset?
Ferrorthorn w/Leftovers
Power Whip GyroBall Ingrain/Leech Seed (Thunderwave or Explosion)
Definitely go with thunder wave. Even though he won't be able to outspeed anything even with thunder wave, it will surely help out your team. This will allow slower sweepers like Hydreigon to destroy. Explosion... well I hate explosion. Especially if your opponent switches to a resistant type, and you kill yourself for no reason. Go for thunder wave. Hope this helps!
I think that you should go with neither. Thunderwave will make Gyro Ball do less damage, while you can make a useful move like Stealth Rock instead. Stealth Rock will especially hurt Fire types who will try to switch in to counter you, and will just hurt other team members.