You could place a Boss Detail at the front location and trigger it off of one of the escorts, so that he'll be waiting for you at the door. Right now there is no way to have a single-critter ambush.
Sure, set BossGuy as a boss objective, and set the surrounding enemy group to "security guards," and set the objective to spawn when you free the ally. If you have an appropriate map, you can have him spawn near where the ally is freed, to make it look like an ambush. There is no way to make an actual ambush have just one BossGuy in it, as far as I know.
Eva Destruction AR/Fire/Munitions Blaster
Darkfire Avenger DM/SD/Body Scrapper
Arc ID#161629 Freaks, Geeks, and Men in Black
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Thank you, I'll give that a shot. 
I tried to create a mission (mission#3 in my newest arc), and completely bumbled it.
The idea was that you would finish an unsuccessful kidnapping by kidnapping the two civilians "escort to door" and rescue the minion ("grab the ally") that the contact had sent to do the job. Upon freeing the minion, who would fight alongside you, a Named Boss hero would show up (Ambush spawned by completing objective) to fight you. Named Boss would be accompanied by a few of the mission's native foes.
Unfortunately, upon freeing the minion from the mission's native foes (Security Guard), he flees for the door. "Oh well", I think, "No big deal. Makes sense for a villain to save his own skin, but I'll fix that later. I'll wait for the BossGuy and his Security Guard escort to show up in 5...4...OUCH!!!!"
I get plastered by a group of 4-5 identical Custom Critter Bosses. It's Bossguy multiplied by setting the ambush to Hard.
What's a better way to do this?
Should I have the Minion AI set to Follow, then change the ambush to "Defeat Boss and Guards: triggered by Free the Ally"?
Is there a way to have the BossGuy show up as only one foe, regardless of teamsize, but perhaps have an escort, similar to robbing the bank in a Mayhem mission?
Defeating BossGuy wouldn't be critical to mission completion either way.
Required Mission Objectives would be "Kidnap 2 civilians, Rescue Minion".