This request is picked up from the Request Board in Galaxy Hall, but it only becomes accessible once you’ve caught or defeated a Croagunk. When you pick up this request, you’ll be prompted to speak to Pesselle, who’s found in the medical room on the ground floor of Galaxy Hall. She’ll then ask you to catch her a Croagunk.
Where to Find Croagunk in Arceus
Croagunks can be found in abundance in the water at Gapejaw Bog in Crimson Fieldlands. They will become hostile on sight so you’ll want to either sneak up or avoid one-shotting one in battle so you can catch it.
Once you have one, head back to Pesselle and hand over the Croagunk. You’ll earn x3 Full Heal and x3 Exp. Candy S.