Any way I can do this?

I've got an arc that I've been constantly updating since I created it. I had this great idea for the final mission, though I don't know how to do it or if it's possible.

So, the final mission has you going to meet up with your enemy for a one-on-one battle. Of course, it turns out to be a trap, and there are tons of enemies waiting for you. I want to have it so that no other enemies appear at all in the map except for one boss which will be placed at the back. After the boss is defeated, the trap is sprung, and all the enemies will populate the map, forcing you to fight your way out.

So, is this currently possible at all? The only work-around I've found is to add as many "defeat boss" details as possible and trigger them all on the trap, which then takes up most of the spawn points. But I don't want to fill my mission with nothing but bosses every mob, since this arc is supposed to be solo friendly.

Set map to empty / no enemy group, and have the maximum amount of patrols spawn when you boss takes damage or whatever is your trap condition?

I think that is, or at least was, the default way to do that.

You can do it the way you described, using a custom minion or lieutenant for the "defeat a boss" detail. Unfortunately you can't use standard minions or lieutenants.

Patrols work as well, except soloers might run into a problem if a few patrols bunch up in one spot.