Challenge help needed: 6 Unique bosses in a Ritual. How to do it???

As the title says I'm at wits end. I tried ritual with the devoted modifier, lured like 10 bosses in, didnt work. Tried with arena and maven node for 3 additional bosses, didnt work. Tried with the 15% chance for a pack to be replaced by mapboss scarab + ritual vessel with a captured boss in it, didnt work.

At this point Im confused about whether this is bugged, I'm unlucky or plain and simple do something wrong.