By Thy Pale Beams I Solitary Rove

Talk to Kujirai Momiji: First things first, you need to find Kujirai Momiji. She's hanging out northwest of Inazuma City, right by the southern waypoint in Byakko Plains. Chat with her, and she'll explain that she's found a clue in her dad's old sailing log about a special lure. Apparently, the bait uses Seagrass and Magical Crystal Ore. She's got the Seagrass, but you'll need to head to Araumi to gather three pieces of the Magical Crystal Ore. She'll also mention that Araumi is crawling with monsters, but don't sweat it – you've got this! She'll tell you she'll meet you west of Araumi later.

Go to Araumi and collect 3 Special Magical Crystal Ore: Now, teleport to the Araumi waypoint, which is up north on the island. From there, glide east. Keep an eye out, and you should spot the Ore nestled near the mountain. Just attack them to collect all three pieces. Once you've got them, head west.

Talk to Kujirai Momiji: Remember how Kujirai said she'd meet you west of Araumi? Keep heading west/northwest from where you collected the ore, and you'll find her waiting by the coast. Talk to her, and after a quick scene, she'll whip up the bait. You'll think you've finally found the Lunar Leviathan, but surprise! It's actually a Ruin Guard.

Defeat the Ruin Guard: Time to show this Ruin Guard who's boss! Since you're right by the ocean, the 'Wet' status effect will be applied automatically. This is perfect for using Cryo attacks to freeze it solid. While it's frozen, go ahead and wail on its head, where its core is, to stun it. You'll take it down in no time!

Talk to Kujirai Momiji: After you've dealt with the Ruin Guard, head back to Kujirai. She's a bit bummed that her dad's bait didn't quite work out as planned, and even Paimon's comforting words can't quite cheer her up. She'll give you your payment for the job, and then she'll leave. And that's it – quest complete!