In ORAS, Trevenant gets a new move that makes it one of the most annoying stallers that is basically guaranteed to take out at least one of the opposing team's Pokemon. This move is Block.
While it might not seem like much, when used in tandem with the move Curse, it can be absolutely deadly.
Trevenant @ Lum Berry
EVs: 252 HP / 4 Def / 252 SpD
IVs: 0 Spe
The strategy is rather simple. If you can outspeed the opponent and use Block, you can proceed to use Curse and hack away at their health, while they are unable to switch out. Because Trevenant's Special Defense is higher than its Defense, Will-o-Wisp can cripple physical attackers and somewhat balance out its defenses. When you get low on health, Rest can heal you to full health and you will be instantly cured thanks to the Lum Berry.
There are, however, a few flaws in this strategy. In Gen VI, Ghost-types can escape even after being hit with Block or Mean Look. As a result, all Ghost-types should be eliminated before bringing in Trevenant.
Trevenant is abysmally slow, though, so Trick Room support is crucial. Rest can be replaced with Trick Room, if you prefer, and a Harvest + Sitrus Berry combo should be used instead.
Taunt shuts this entire set down, so it's a VERY good idea to get rid of all the opposing Taunt users before bringing Trevenant's in.
All in all, this is a set that does require a lot of setup, but if done correctly, has a high payoff.