Walkthrough

1. After speaking to Professor Tangrowth about how to wake up Mosslax, talk to Meowth, as it may have an idea of how to start brightening the place up.
2. When you talk to Meowth, it will suggest lighting up the streetlights around the area. To do this, select a Pokémon in your party that has the Generate specialty and instruct it to light up several streetlights nearby.
3. Run back to the cave with Meowth to see if the streetlights helped Mosslax awaken. However, it has not, but some flowers will bloom on its head to signify how soon it will wake up.
4. Professor Tangrowth will calculate the amount of brightness you have accumulated. You will need 100 points to fully awaken Mosslax.
5. This time, Torchic will have an idea. Speak to it, and it will suggest using fire to brighten up the place.
6. Set up the newly unlocked Resort Meal Prep habitat by placing each item correctly: put a chair, a campfire, and a plate with some food on it under a palm tree along the shore. Check with Torchic after you finish setting up all three items.
7. Go back to the cave and speak with Professor Tangrowth about Mosslax's progress, and he will inform you that there are still some ways to go.
8. He will bring up a Pokémon that can use electricity to further liven up the place. Just then, a commotion will be heard outside, so go and check it out.
9. Follow all the Pokemon running towards the unconcious creature on an island and speak to it to discover a rather pale-looking Pikachu, which will be nicknamed Peakychu.
10. Without any recollection of how it got there and having just woken up, Peakychu won't be able to use electricity like an ordinary Pikachu. However, it may still find other ways to help.
11. Wait for Peakychu to reach the cave and speak to it and Professor Tangrowth for more ideas on brightening up the place.
12. They will suggest talking to the other Pokémon to see if anyone has any other ideas.
13. Find Peakychu and Professor Tangrowth by the windmill, located right in front of Mosslax's cave, to learn about power sources, such as windmills, and how to distribute power via utility poles.
14. Speak to Piplup, and it will lead you to a waterwheel to the east of Mosslax's cave. You will then be introduced to power sources, such as waterwheels, and learn how to connect to them via utility poles.
15. Activate waterwheels by searching for water sources that connect to the canals they are in, and carving a channel between them to have the water run through the wheel and make it turn.
16. Trubbish will be right outside the cave, looking at something in the distance. Speak to it, and it will point out the large building a little to the east of the cave. Go ahead and check it out together.
17. Don't forget to bring a Pokemon with the Generate specialty or connect utility poles from power sources all the way up to the lighthouses. Be sure to light up all four spotlights to fully activate it!
18. Torchic will have its own idea. Speak to it, and it will lead you to the town's furnace, where you can put burnable objects to be incinerated to generate power.
19. Notice the light bulb at the top of the furnace. Connect to it via utility poles to distribute power to different parts of the town.
20. Speak to Professor Tangrowth and Peakychu for more ideas on how to brighten up the place.
21. Craft some seashell lamps and plant them all over Mosslax's cave to help brighten up its surroundings. Be sure to connect these lamps to a utility pole to turn them on!
22. Go and check on Peakychu by the strange device, a little to the west of Mosslax's cave. It will suggest connecting the device to power sources nearby using utility poles.
23. This charging station has two nodes on either side, and five bulbs right above it to signify its power capacity. To charge up Peakychu, you'll need to have all 5 bulbs lit up. Once they are, speak to Peakychu to charge it up.
24. Once Peakychu has fully charged up, speak to it to unleash its power and completely brighten up the entire resort.
25. With things as bright as they could be, return to Mosslax's cave and see if it has awakened yet. But even after all your efforts and all of its flowers bloomed, it still wouldn't open its eyes.
26. Just then, thunder will part the thick clouds, strike the cave, and carve out its roof, forcing sunlight into the enclosure. Professor Tangrowth will point out a Pokémon high up above the cave that may have been responsible for the thunderstrike.
27. Make your way to the hut directly above the cave to find the legendary Pokemon, Raikou. Speak to it to learn more about the town's history, and for an apology for hitting Mosslax with its thunder.
28. This will unlock Raikou's entry in the Pokedex.
29. Inform Professor Tangrowth about your findings and your conversation with Raikou. With all of that out of the way, speak to the now-awakened Mosslax to conclude the request!

Walkthrough source: Game8.