Okay, your main character's team has to consist of mainly water-type Pokemon to defeat Golem, who's rock and ground-type. For example, your main character in your story should have an Oshawott, which is mostly water-type and during the battle, it evolves into Dewott. The Dewott uses Razor Shell on the Golem and the Golem faints. Your main character wins the battle. I think that story would be a little bit boring, so make sure the Oshawott gets attacked by Golem and it's about to faint, but it then evolves into Dewott. Dewott uses Razor Shell on Golem and it faints from exhaustion.