I was told to level it up in the rain and did, but it did not evolve. I checked again and leveled it up again, still no evolving. How do I evolve him?

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2 Answers

Sliggoo evolves into Goodra starting at level 50 if it is raining in the overworld. Rain caused by Rain Dance or Drizzle will not trigger the evolution.

Here are some places where you can find overworld rain:

Sliggoo evolves when levelling up during naturally occurring rain starting at level 50.