HOW TO CLEAR RAIN'S REQUEST
This is a pretty easy Sub Quest, talk to Rain in our expanded home and she'll ask the kill the Wanderer Eater in Remnant Wasteland, a level 270 HNM (the highest level so far for an enemy).
It's a scorpion type enemy so just keep hitting his weak spot, it might take a few minutes but he'll eventually die. No big rewards for this quest, just Rain's gratitude.
For a detailed walkthrough check the video below: