Obtained: Speak to the Masked Salesclerk behind Door 3 in the Daiba Studio Idolasphere.
You will need to defeat 4 Regal Knights to find the Scarlet Armor. *Best spawn point for the Regal Knights was on the walkway near Door 4. After you have obtained all the items, head back and speak with her at Hee Ho Mart in Shibuya to complete the request.
Remark: Melmark x3 and Attack Mirror