You cannot. The Smogon tiers system is merely fanmade, Nintendo or Game Freak have not implemented them in the games. However, if you are batteling a friend or another person using the Friend Codes, you can communicate with, you can decide if you are going to use a certain tier.
But there is no way off batteling in a specific tier in Random Matchup, PWT or any other ingame battle so to say.
You can't because tiers are fan-made so, Game Freak wouldn't implement them in the games.