Delphox vs Serperior 7-Star Tera Raid : Foxy Victory

No commentary - All the battle, none of the talk.
Serperior is a contrary user, so it's set up to boost itself with its own debufs...
However, Delphox was my starter in Pokemon X/Y and I know she's got the solution to that!
Skill swap, then lower its Sp Atk, then Tera, then raise your own Sp Atk til it uses Gastro acid, then back to the first move til it's down. You'll see.
Try to get one of the NPC teams with Gardevoir or Arboliva and if he does Gastro-acid at the beginning of the fight, it will go slower; so don't feel bad about resetting the run. It's an easy fight overall once contrary is negated.
I was able to catch him on the second try, so the timing of my moves can be further improved. Your runs might be even better than mine! I hope it works for you.
Get out there and WIN fam!