There aren’t many strong Steel type Pokémon, but Registeel is certainly the most dangerous. This Legendary Titan can dish out the damage.
Registeel is a pure Steel type Pokémon that can know one of the most powerful moves in Pokémon GO, and that is the Normal type move, Hyper Beam. Even if Registeel knows that move, you can defeat it with the right Pokémon. Fighting, Fire, and Ground type Pokémon are what you want to have in your lineup.
The Legendary Beast Entei will be your best counter against Registeel. While known for being able to take damage, Entei will be able to dish it out even more with being one of the stronger Fire types. Fire Spin and Overheat are the moves you want.
The Fighting type Pokémon is surely going to be the champion in your lineup against Registeel. Counter and Dynamic Punch are the moves you want Machamp to have.
The Ground type Legendary is going to be a monster against Registeel due to knowing dual Ground type moves. You’ll be able to put out a high amount of damage with Mud Shot and Earthquake as Groudon’s moves.
This Fire and Fighting type Pokémon has always been interesting to me and I think is a little under-powered in Pokémon GO. The moves you want Blaziken to have are Counter and Overheat.
The Fire type Eevee evolution Flareon will be a must to have due to how much damage output you will be able to do. Fire Spin and Overheat are the moves you want.
With a little work and a lot of luck, you’ll be able to add Registeel to your PokéDex. Some of these Pokémon will also be great against Regice and Regirock as well.