With my EV spread, it still gets the speed boost even with a naughty nature. It's offensive stats are 191 attack, 175 special attack, and 201 speedTapu Lele is Pheromosa's best friend. Psychic Terrain stops priority moves which often is how snowballing Pheromosa's are killed.
I would recommend Naive Mosa over Naughty to get the speed increase after KO's thus making Phero almost always go first under Psychic terrain
And I definately agree with adding tapu lele to the team to stop priority