It depends on your team composition and what tier you intend to battle in. I am guessing based off my knowledge of the metagames that Trevenant is likely going to be NU. There are just so many Grass types that I think perform better than it overall, such as Amoonguss and Tangrowth. I also believe that Gourgeist is the better Pokemon of the Grass/Ghost types, as Gourgeist has far greater bulk. The advantage of Trevenant though is that it has higher power STAB attacks than Gourgeist, knowing Wood Hammer and Shadow Claw. If you need a more offensive counter to Fighting, Ground, and Water, Trevenant would probably be the better choice, and a solid teammate in the lower tiers.
Natural Cure is less useful than it was in past generations, at least on a Grass type. This is because Grass now has an immunity to Stun Spore, Spore, and Sleep Powder, meaning there are less status moves that can cause Trevenant grief. It can however be used to provide Trevenant with some form of recovery with a more reliable Rest, although Rest+Natural Cure is still not as good as a standard recovery move like Recover, mainly because of how predictable you become as soon as you use Rest. Frisk is a surprisingly overlooked ability, mainly because the Pokemon who have it as an ability just aren't very good or have some of the best abilities in the game, such as Gothitelle with Shadow Tag and Yanmega with Speed Boost. But the benefit of Frisk is great indeed. It allows you to scout out your opponent's item, which in many cases will give you the set the opponent is using, allowing you to better handle your opponent's team. I would actually advocate the use of Frisk, crazy as it sounds. As for set, this is the one I've thought of:
Trevenant @ Leftovers
Ability: Frisk
EVs: 252 Atk / 252 HP / 4 Spd
Adamant Nature
- Wood Hammer
- Shadow Claw
- Rock Slide
- Leech Seed/Will-o-wisp
A fairly offensive set, Wood Hammer and Shadow Claw are STAB. Grass/Ghost is not very good offensive typing, and it leaves Trevenant unable to really do much to Flying types in the tier, such as the popular Braviary. That's where Rock Slide comes in. Leech Seed is used to help alleviate the recoil of Wood Hammer, and also provides teammates with some healing if you use Trevenant in an offensive pivot role. Will-o-wisp can be used though in order to neutralize physical attackers and provide Trevenant with more bulk.
If you actually mean Phantump, as in Little Cup, I might suggest considering its use, because the typing is actually pretty good in the current Little Cup metagame, as Fighting immunity is pretty big. Not going to be good as Misdreavus though, or Ghastly.Main draw is Harvest+Oran Berry. If you had to go either Natural Cure or Frisk though, I'd go with Natural Cure, as I tend to find items to be fairly predictable in Little Cup.
Phantump @ Oran Berry
Ability: Harvest
Level: 5
EVs: 252 HP / 196 Atk / 52 Def
Adamant Nature
- Wood Hammer
- Shadow Claw
- Rock Slide
- Will-O-Wisp
Pretty much the same as the Trevenant set, however Will-o-wisp is more of a must to stop the Physical attackers of the metagame.
