My opponent has a Pyroar and when it's in play he doesn't place any other pokemon on his bench so I can't have him switch out. Is that allowed?
Yes. The rules only require that the player have one Pokemon in the active position. Pokemon on the bench are optional. If the opponents active Pokemon is knocked out, and they have no pokemon on the bench to replace it, the attacking player wins.
Pyroar has an ability, Intimidating Mane, which reads "Prevent all damage done to this Pokémon by attacks from your opponent's Basic Pokémon."
It is up to you to build a deck that includes an appropriate counter to the opponents Pyroar. This could be Rayquaza's Shred attack, Golurk's Devolution Punch, Staraptor's Strong Breeze, or any other pokemon who's attack reads "This attack's damage isn't affected by any effects on the Defending Pokémon"
Or you can evolve a Stage 1 or 2 Pokemon, or Mega EX Pokemon, who's attacks aren't blocked by Pyroar's ability.