The Pyramid Chief in Battle Frontier has the following Pokemon-

1. Regirock
2. Regice
3. Registeel

Be careful of the Regirock, it can use Explosion on its first chance so I would advise you to have a Pokemon that knows Dig, Dive, Fly, etc. The point is thatwhen the Regirock uses Explosion it misses your Pokemon.

I'm not sure about the fact that he has a Registeel but as I battled his Regirock and Regice ( I lost to the Regice by the way ) I'm assuming his third Pokemon is a Registeel.

I would suggest you take Fighting Pokemon for him.