I just found the easiest strategy to beat Solaris. You need Banette and Shiinotic both lv45, a quick claw, and a focus sash.
Send out Banette first,
- use will-o-wisp, quick claw must activate
- Garchomp must use fire fang so you survive
- Solaris will use full heal so just use will-o-wisp again
- select will-o-wisp again just in case but for me he just KO'd me which didn't matter
Send out Shiinotic,
- use strength sap
- spam till lowest attack
- spam moonblast until Garchomp goes down
You just beat Solaris with two pokemon and 3 moves lmao