Because Kabu is the third Pokemon Gym Leader found in Pokemon Sword & Shield, he will definitely give players a much harder time than both Milo and Nessa before him. All three of his Pokemon will be fully evolved when you challenge his Pokemon Gym, making it reasonable that most trainers quit the Gym Challenge when they can't defeat him. However, Kabu can be much more manageable to defeat should you make smart decisions before facing him. For one, if you select Sobble as your Starter Pokemon, Kabu is so much easier to handle since his Pokemon are extremely weak to Water Type Attacks.
Even if you don't select Sobble to journey through The Galar Region with, there are still a few things one can do leading up to the Motostoke Pokemon Gym so Kabu isn't too hard to defeat. One of these things is to capture Magikarp by fishing on the routes with water before challenging the Motostoke Pokemon Gym. Magikarp evolves into Gyarados at Level 20, and Gyarados is one of the strongest Water Type Pokemon you can have. Again, Kabu's Pokemon are extremely weak to Water Type Attacks.
But should you not want to start with Sobble or capture and evolve Magikarp in preparations to facing Kabu, you don't actually have to in order to defeat him. Kabu's three Pokemon will all be below Level 30, so one could, honestly, train all of their Pokemon to Level 35 or Level 40 and still defeat Kabu. 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 early in the game.
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