Unofficial Fallout 4 Patch

A script fragment on DN009_MQ205 reduces the health of all gunner prisoners to 5% before any of the scenes start running. Plus, the courser has a perk to deal double damage anyway. So I ran a few preview checks on the leveled list that generates his weapon, and the best weapon he can get even at higher levels deals 37 energy damage in a single shot. Some weapons are automatic, but this doesnt matter here because he's scripted to fire single shots at his gunner captives. With his perk, that makes a maximum damage of 74, while a high level gunner's health at 5% is still 65. And we have not even considered the damage reduction by the gunners' armors and their faction-specific resistances (they have their own perks).

So yes, it's likely getting narrow,

And while we are at it, I have seen in my own game that the mysterious stranger - unlike the perk description claims - is unable to kill a high level enemy in one shot either, but that's a different issue and will eventually get its own ticket.

I'll let DN009_MQ205 give him a custom perk that deals a generous extra damage to the faction the gunner captives are in. Can't do this directly on the actor record because he's templated and draws a level-dependent set of perks from a leveled character list.