If you are fighting Steelix in the Rocky field,which i'm certain you do,then Earthquake is useless.When using earthquake the field adds bonus Rock damage,which pretty much doesn't help at all.You could use a Scrafty with Intimidate and use High Jump Kick till it dies.If you can teach Donphan Bulldoze (or Graveller/Golem) do it,since it' not affected by the field plus you lower Steelix's speed.