Pokémon Ai

I recently played through a HC monotype poison nuzlocke and was completely lost as to the ai. I was battling Serena after the 5th gym and I had my Nick king out at half health and I thought I outsped but I actually didn’t and should have been killed by psybeam but the delphox went for mystical fire. I’m wondering why the delphox didn’t go for the kill and hoenslty why it was only using mystical fire despite having a super effective move against my whole team🤷‍♂️