A wild sheep chase help and suggestions

Hello, I have a dilemma that I am hoping y'all can answer. You see, there's a new player that will be joining my group. And I want to host it and play the famous one-shot: a wild sheep chase. The game will be accompanied with my old players. But, knowing my old players.. they're just gonna build an unfair op character that overwhelms the story. I want the new player to experience the game entirely and not just watch the other players wipe the whole enemies in one swoop. Do y'all have any suggestions or guidance to provide to make the game enjoyable and fair to the new player? Thank you so much for the answers!!