The best way to defeat Nessa is to go to the Wild Area and farm EXP Candies from Max Raids, and give enough to Toxel to let it evolve at Level 30. Teach it Shock Wave, and that plus the level advantage will make it easy to defeat her. Make sure to get some Revives, so if Toxtricity somehow falls, you can just bring it right back to the battle.
Source: How I defeated her when I had a Toxel