Best thing to do is hit with a really powerful special attack. It doesn't have the best defenses so any really powerful attack can KO it.
Depepending on your starter you can use their special attack. So Frenzy Plant with Grass starters, Hydro Cannon with Water starters, or Blast Burn with Fire Starters.
This will KO Sableye and then you can switch out so that you don't have to recharge the next turn.