I'm doing it with my friends same problem tho rng is just terribleI didn't have a hard time getting it at all when I went through CoA with my group.
We have an understanding of what armor type each player wears, or we will discus with each other what we are looking for in the dungeon before going in. If we find a weapon/armor piece for the set someone wants, when we are out of combat, we will just give it to the person who is looking for it. For example, on my DK, I was looking for Burning Spellweave, I only found one piece of it. Two friends in my group found about 3 pieces each of it, so they gave it all to me. Now, what I did find were pieces of the sets they were looking for, so I gave the pieces to them.
With just our first run and the pieces they game me, I had 5 out of 5 pieces needed for the set.