Did it on expert, we used Rein, Orisa, Bastion, Zen.
The eternally never nerfed Rein damage is perfect for this, enemies bunch up naturally and he just swipes them, orisa is the main shield cuz she can shoot while shielding unlike rein (still useful even if she does like 1/10 of his insane fucking OP damage) and bastion just mows shit down for breakfast. Zen is useful for being able to heal, debuff and deal dmg at the same time, which mercy can't do, and since everybody has the ability to rez, she becomes a bit obsolete.
I'm surprised anyone has trouble on normal though, that's a bit weird to me unless you run with no tank/heals, I don't know what to say.
EDIT: forgot to mention I made it after several attempts, expert felt like 5 steps harder than hard difficulty. The gap between hard and expert is gigantic.