It's an energy sink, as with most of the missions. Just hold your nose and suffer through it.

I've been having the same issue. I'm stuck also, cleared the rest. Mine's for "2* or higher ATK gloves, chest and boots on an attack character". I've put many of these sets on my characters. More than the 2 out of 3 I've been credited. It will be a "silver sink" removing them later. Could there be a bug? Holding our noses doesn't clear the challenge so we can get the rewards and move on to more advanced artifact challenges. While I agree that they are "energy sinks", there are rewards here both now and later.  

Yours is different. You are probably using ATK items, instead of ATK% items.