Slick Skull Delivery is a request which is available after starting Chapter 2 in Tokyo Mirage Session #FE Encore for Nintendo Switch. It is repeatable.
Slick Skull Delivery Guide - Tokyo Mirage Sessions Encore
Slick Skull Delivery Guide
Illusory Daitama
Talk to the Masked Salesclerk in the Idolasphere in Daitama. You can find him right after you take the first elevator down to the lower level. He gives you a Slick Skull and wants you to find 4 Slick Skulls from Dark Mages in this dungeon. Run around and fight Dark Mages until you found 4 Slick Skulls.
Shibuya
Go to the Hee-Ho Mart in Shibuya and give the 5 Slick Skulls to the Masked Salesclerk to receive your reward of 1 Melmark and 1 Repel Water.