Weeeelll...I'm no expert of using Trevenant, despite my love for unorthodox strategies because frankly, Trevenant has almost nothing to make it stand out. Everything it does can be considered niche. But since I feel like the POTW discussion will need at least one actual set, obvious or no, here's one blatantly (and lovingly) ripped off from Smogon:
Trevenant @ Choice Band
Natural Cure
252 Atk / 4 SpD / 252 Spe, Adamant or Jolly
- Wood Hammer
- Shadow Claw
- Earthquake
- Horn Leech / Rock Slide
This set is a weird mix of roles, but situationally it might be nice to have around. First of all, the nature: choose it based on the moves. I recommend Adamant with Horn Leech and Jolly with Rock Slide; being slow is slightly less of an issue with a healing move, but you'll want the coverage with Jolly. Next, the item: Choice Band is best, and that's about it. Harvest might work on a sun team, but even then there's a lot of holes in it (see: Knock Off). Natural Cure is Trevenant's best ability most of the time, so when you switch out because of Choice-locking you'll ditch status. The moves, then, are pretty basic for a Choice set: the best coverage and strongest moves available. Wood Hammer hits quite hard off of Trevenant's Attack with a Choice Band, as does Shadow Claw (having less power but no recoil and better neutral coverage). Earthquake hits Poison- and Steel-types that shrug off Trevenant's STABs. The last slot is up to you: Adamant nature means decent healing off of Horn Leech and a reduced need for coverage, but Jolly makes better use of Rock Slide's excellent coverage with Earthquake. Flinching probably still won't happen though. Threats this set checks? Not too many; Gastrodon, some Musharna, the rarely seen Wartortle, and a handful of others. Checks to this set? Weezing, Skuntank, Scyther, Archeops, Pyroar, Haunter, the list goes on. Trevenant isn't PU; it has a solid defensive typing, a decent STAB combo with access to good coverage moves, a nice 110 base Attack, and it handles status rather nicely, but it is often outclassed. In an environment full of min-maxing, Trevenant performs several tasks well but none perfectly, and it shows.