Getting a boss to spawn solo

Hiya,
In a defeat a boss objective, I've got a custom boss who is supposed to spawn alone. In the objective settings, I've set his surrounding enemy group to "empty." The default enemy group on the map is Council, and the boss is spawning with Council friends around him despite being set to Empty. Does anyone know how I can get him to spawn with no enemies in his group?

I also tried putting the boss in a custom group by himself -- one that's otherwise empty. In this case, even on "easy" spawn mode, the boss spawns with 2 other copies of himself.

Is it possible to get a boss to spawn alone?

in the objective with the boss spawn, if you go into boss settings theres a little section that says "surrounding enemy group" which is normally selected in the dropdown as "same as boss" but going into that you could choose to set any other enemy group (normal or custom) or selecting "empty" to make him spawn alone

Thank you. Unfortunately, when I set the surrounding group to "empty," the boss spawns with Council minions anyway (the map's main group). I'm confounded as to why. :/

Create a 'rescue captive' spawn with Single enemy, it auto-completes.

Create an ally with Single and betray when first objective completes... you now have an 'ally' that will aggro the player the instant the player is close enough.