If I see it honestly, a good recovery item paired with blade of vampire could have amplified even more the dmg easily (overall good build aka instead of using to much soulboon for dmg to rely better on a better offensive)
rifle third skill makes the bullet richochet between enemies, however every hit the damage reduced. you can increase it with damage modifier and buff increase soul boon
From the real mirage you have a chance to get it. With active pandemonium, this chance increases. So if you go to Pandemonium 2 your chance will be much greater.
This video, if I'm not mistaken, was in the true mirage without the active pandemonium. But later I tested it in pandemonium 2 in the true mirage and it was one of the best builds to do this mode.