Try this moveset:
Earthquake: Deals with Fire types, high power, high accuracy, double damage against foes using Dig.
Aerial Ace: Decent Power, doesn't miss, used to deal with Fighting types.
Ice Punch: Decent power, high accuracy, chance to Freeze, used to deal with Ground types.
Flash Cannon is good unless you have higher Attack, then get in Iron Head.
Edit: Swampert is right. Replace Flash Cannon with Gyro Ball. You could keep Aerial Ace or get in Hammer Arm, your choice.
Edited Edit: Never mind if Registeel can't learn Gyro Ball.