I think what you need is at least 3 (I think) Fully Trained Pokes in your party. And by Fully Trained, I mean all EVs maxed out. If that doesn't work, then I guess waiting is the only option. Or, it could be that each appears on differnent days, such as Terrakion appears only on Saturday and Tuesday (guesses, not actually their days).
Source: Experience and Serebii