Since Nessa is just the second Pokemon Gym Leader you will face in Pokemon Sword & Shield, she will also make for a pretty easy defeat. Her Drednaw might give some people a hard time, but this can easily be avoided by making smart decisions before facing the Hulbury Gym Leader. For one, if you select Grookey as your Starter Pokemon, Nessa is extremely easy to handle since her Pokemon are extremely weak to Grass Type Attacks, especially her Drednaw that has a x4 Weakness to Grass Moves.
However, even if you don't select Grookey to journey through The Galar Region with, Nessa can still be handled if, as mentioned, you make the correct choices leading up to her Pokemon Gym. One thing you can do in this case is capture Seedot (Sword) or Lotad (Shield) on Route 2, which comes up pretty early in the games. Both Pokemon actully learn the Grass Type Attacks Absorb and Mega Drain by Level 15, which will do loads of damage to Nessa's Water Type Pokemon. Both Pokemon also evolve at Level 14, which means you'll have an even stronger Grass Type Pokemon to face Nessa with!
But should you not want to start with Grookey or capture Seedot/Lotad in preparations to facing Nessa, you don't actually have to in order to defeat her. Nessa's three Pokemon will all be below Level 25, so one could, honestly, train all of their Pokemon to Level 30 and still easily defeat Nessa. Again, this is incredibly simple to do if you participate in plenty of Max Raid Battles in the Wild Area and stock up on EXP Candy.
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