The level scales to the available range that the enemies are in.
If you want a low level mission with baddies only being from level 1- 14 include some skulls, who don't spawn above 14. Outcasts are limited to level 20. ect ect
"I accidently killed Synapse, do we need to restart the mission?" - The Oldest One on Lord Recluses Strike Force
That is what I needed to know. Thank you.
And I think you can include those guys anywhere: as allies, as hostages, as bosses, whatever. It'll constrain the level range no matter where they appear, even if they don't spawn.
For instance: make an ambush of Skulls that spawns upon the rescue of a hostage that is held by friendlies. You will never rescue that hostage, so the Skulls never appear, but the mission level is constrained anyway.
Good to know.
I was intending to cap a two part mission I'm creating to give a 10-20 level player something to identify and try out as well as something to solo.
I'm not sure if it will be appreciated, but I figured why not test it out. 
Also it's not just the enemy group you include. I included, for example, a specific mob as ally (Longbow Guardian) to limit the range of one of my arcs to 1-9, even though Longbow go from 1 to 54.
If you want a wider range, create a custom group with a few mobs from the min to the max range you want, and add them somewhere. I did that with PPD cops to make a 5-25 range.
Players' Choice Awards: Best Dual-Origin Level Range Arc!
It's a new era, the era of the Mission Architect. Can you save the Universe from...
The Invasion of the Bikini-clad Samurai Vampiresses from Outer Space? - Arc ID 61013
I'd like to make missions intended for lower levels (like 6 to 15), but I seem to be capped at level 20 to 29...is this deliberate or am I doing something wrong?
All my villains are tagged as 1-54. But I dont see any way to define a level range...does this mean anyone 1 to 54 can play the mission and it will scale to their level automatically?