Are you sure the champs are ATK champs?

Are you sure the primary stat of the items is ATK%, and not just flat ATK?

Are you sure the items are level 12 or above?

Yes to all the above.

Although I'm not entirely sure if my understanding of ATK is correct regarding champs.  My Attack champs have the word "Attack" but I have only seen the word ATK on gear and in the mission statement.  Should I see  ATK somewhere on a champs somewhere?

I first tried with this set of misc gear then with a set I farmed. 

i

You can find it here when looking at your champ: 

i

That's what I thought.  I am using an attack champ.

i

Verify all three conditions again. Post screenshots of the items and the champs, if necessary.

OK I farmed another set of ATK% to meet the mission requirements.  Should I level them to 12 before equipping them?  I'm also going to put them on a champ that has never had gear before.

Posting here for verification before I waste any more silver and time.

i

yes, level them BEFORE equiping

It finally worked.  For the benefit of others here is what I experienced:

I tried using some existing 12+ ATK% gear that I already had.  It is possible (likely) some of it may have already been put on a champ at some point.  This did not work.

I then farmed a new set of ATK% gear and equipped it onto the same champ I had tried previously.  That did not work.

Final approach:   Farmed a new set of ATK% gear and equipped it onto a new champ.

They really ought to fix this bug or get rid of this quest altogether.  I wasted at least an hour and 2-3 refills of energy trying to farm pieces worth keeping, before i gave up and just used trash.