Since Raihan is the eighth and final Pokemon Gym Leader found in both Pokemon Sword & Shield, defeating him will be nothing short of tough. Like a number of the Gym Leaders before him, all of Raihan's Pokemon will be fully evolved. If that's not bad enough, he will also bring four Pokemon into battle, using Weather Effects to his advantage throughout the match.
Thankfully, like all Gym Leaders, Raihan can be much more manageable to defeat should you make smart decisions before leading up to battling him. One thing you can do is choose Sobble as your Starter Pokemon way back when your journey first begins and fully evolve it by the time you reach Raihan. Sobble's Evolution Line might not learn any Ice Type Moves simply by leveling up, but they can learn a number of strong Ice Moves by using both TM's and TR's. Sobble's Evolution Chain also learns plenty of Water Type Moves naturally. If Raihan has an Achilles' heel, it's definitely found in Water Type Attacks and Ice Type Moves.
Even if you don't select Scobble to journey through The Galar Region with, there are still a few things one can do leading up to the Hammerlocke Pokemon Gym to better deal with Raihan. One of these things is to capture Applin back when you first travel through Route 5 and evolve it using the Tart/Sweet Apple. Both of Applin's Evolutions are Grass/Dragon Type Pokemon, which should fare well against Raihan's Ground Types (weak against Grass Type Attacks) and Dragon Types (weak against Dragon Type Attacks).
But should you not choose Sobble as your Starter Pokemon or have the desire to catch and evolve Applin, like all Pokemon Gyms you don't actually have to in order to defeat him. Raihan's four Pokemon will all be below Level 50, so one could, honestly, train all of their Pokemon to Level 50 or Level 55 and still defeat him. This is incredibly simple to do if you battle all the trainers throughout your journey and participate in plenty of Max Raid Battles back in the Wild Area (and stock up on EXP Candy when you do).
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