![Shiny Trevenant][1]
If you insist on a ChestoRest Trevenant then...
Trevenant@Chesto Berry
Ability: Harvest
EVs: 252 ATK, 128 DEF, 128 SP.DEF
Nature: Brave (+ATK, -SPE)
Characteristic: Likes to thrash about (+ATK, 31IVs)
Moveset:
Rest - Restore all health without wasting a turn(s) sleeping because of a Chesto Berry which can be harvested [30%].
Will-O-Wisp - Damages the opponent even when a turn is spent resting, also reduces the opponents attack stat by 50% enabling Trevenant to take more and bigger hits.
Wood Hammer - Extremely powerful STAB grass attack, recoil damage doesn't matter that much because of the ChestoRest combination.
Shadow Claw - Powerful STAB ghost move with a high critical-hit ratio (1/8).
I suggest having him/her on a Trick Room team so you can invest IVs and EVs in ATK rather than HP.
![Shiny Trevenant][2]
If you want Trevenant on a normal team, however, then...
Trevenant@Chesto Berry
Ability: Harvest
EVs: 252 HP, 252 DEF, 4 ATK
Nature: Impish (+DEF, -SP.ATK)
Characteristic: Capable of taking hits (+DEF, 31IVs)
Moveset:
Rest - Restore all health without wasting a turn(s) sleeping because of a Chesto Berry which can be harvested [30%].
Will-O-Wisp - Damages the opponent even when a turn is spent resting, also reduces the opponents attack stat by 50% enabling Trevenant to take more and bigger hits.
Horn Leech - Powerful STAB grass attack that further restores HP.
Shadow Claw - Powerful STAB ghost move with a high critical-hit ratio (1/8).
![Shiny Trevenant][3]
A Sitrus Berry and Leech Seed combination will not waste moves by resting; in addition it damages the opponent.
Trevenant@Sitrus Berry
Ability: Harvest
EVs: 252 ATK, 128 HP, 128 DEF
Nature: Adamant (+ATK, -SP.ATK)
Characteristic: Capable of taking hits (+DEF, 31IVs)
Moveset:
Leech Seed - 1/18th of the opponent health is drained and absorbed by Trevenant, damaging them whilst healing you every turn.
Will-O-Wisp - Damages the opponent even when a turn is spent resting, also reduces the opponents attack stat by 50% enabling Trevenant to take more and bigger hits.
Horn Leech - Powerful STAB grass attack that further restores HP.
Shadow Claw - Powerful STAB ghost move with a high critical-hit ratio (1/8).
[1]: http://www.smogon.com/media/sprites/xy/asfront/trevenant.gif
[2]: http://www.smogon.com/media/sprites/xy/asfront/trevenant.gif
[3]: http://www.smogon.com/media/sprites/xy/asfront/trevenant.gif