Using the Kin artifact, it should be possible to get a level where every enemy is a boss. And I do remember getting that to happen at least once, but I don't remember under what conditions. Is it not possible to do this in multiplayer? Is there a time aspect to this? Or are there other conditions I'm not aware of that need to be met, other than having Kin active?
Bosses have higher chance to spawn on higher difficulty (source):
Bosses in Risk of Rain also vary from level and area. Bosses usually spawn at a random rate which is affected by both difficulty setting and timed difficulty of the session.
It's also possible to loop through game levels instead of going to final level via Divine Teleporter:
The Divine Teleporter is a teleporter that replaces the regular teleporter at the Temple of the Elders and all levels after it. There are two options at a divine teleporter. You have the option to "Loop" back through the previous levels and farm more items, or continue to the final level
So you basically need to equip Kin, wait till difficulty rises, then loop through levels until desired boss spawns!