Well said, Elvarion.
I made the huge mistake of selling the atk% items after the first farm marathon from the challenges on the sewers, as I had not seen that those will become a requirement later on and they were low ranked.
Having made the 2nd farm marathon today, it took me roughly 100 runs to get the atk% gauntlets, plate and boots.
This is not taking into account the item rank. I got 2* Gauntlets, 4* Chestplate and 2* Boots from the sewers on hard.
Now it asks to get the 3 items on the lowest difficulty at max rank (which has probably the lowest chances to get) with the rarest attribute. Farming is okay, but that's a bit overkill. Took me 3 hours of constantly pushing the replay button to get just the Gauntlets with atk% and again that was on lowest rank at that difficulty.
I deem the chances as high as getting a legendary champion out of a rare shard - close to none.
Worthy of improvement.
Btw: why sewers on normal at Artifact Challenge 6/10, when most players by then should be close to finishing normal difficulty anyway?