As part of the Expedition to the Barrier Peaks campaign, my PCs took on their first boss. 8 PCs and 4 Henchmen/NPCs with an average level of 6. The boss was a Rubik-like cube robot/mechanical entity called Rethiarius. Corrupted by a creature known as a Telepath (a reskinned mind flayer, inspired by Prey 2017). The cube fight was inspired by similar enemies in Skyforge I encountered back in 2017/18. I used zone combat for the fight, and it worked very well. I am using World Anvil to run my games now, and you can find the boss statblock here and the zone set up for the fight here.