As you wish, you only have one attack move, if you fight against fire or steel, Phantom Force will be a good use.
Also Phantom Force makes you ''dissapear'' for 1 turn and your oponente will get damaged by the burn.
Substitute can be a great choice to. Sub + Leech Seed + Will-O-Wisp + Harvest + Sitrus Berry is a great combination.
So if you want a tank Trevenant you can use Substitute, if you want to get ride off some pokemons that Horn Leech can get ride off, you may use Phantom Force.
In my opinion Phantom Force is better.