I am not sure if it is, but I saw a few movesets that used it and I am just a bit curious if it is possible. I know it's not a egg move in the game but I rlly like that move lol.
The ONLY way to get a Soft-Boiled Cleffa, Clefairy or Clefable is to import one from FireRed, LeafGreen or Emerald. There is no way for them to obtain the move in Sword and Shield, or any other game.
If you can't transfer one you can teach it moonlight which is inferior to softboiled, but an ok alternative. It's not an egg move.
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