I believe that this has been discussed before. Seems like the forum want you to have 2 full demon sets (If you dont have any demon items, it says 34 more items)
I talked to Dragon, this is apparently the case.
I would have tried that previously, but didn't find anyone that would do that which I could trust really. Anyhow, I think Duck is right about this.
Is there any way that I could get this achievement though? Or do I need to collect a second full demon .-.