Mission completion on entrance
Are you sure both hostages are guarded by an enemy and not set to "single", as that setting means that the ally is left alone and therefore, needs no rescuing.
A Penny For Your Thoughts #348691 <- Dev's Choice'd by Dr. Aeon!
Submit your MA arc for review & my arcs thread
Yeah, can't do that in MA right now. Hopefully they'll fix that oversight in the next MA update.
There was a thread on this not long ago, you can check it out here if you want to try a workaround for the time being.
A Penny For Your Thoughts #348691 <- Dev's Choice'd by Dr. Aeon!
Submit your MA arc for review & my arcs thread
I might try creating a separate group containing only the one enemy you want to capture GW, and then set that enemy to spawn uniquely in the group.

<《 New Colchis / Guides / Mission Architect 》>
"At what point do we say, 'You're mucking with our myths'?" - Harlan Ellison
Since when can you do that? Last time I checked not being able to have precisely one of anything in a group was one biggest problems of storytelling via MA.
OK, I've never made an AE arc before but everything in my arc is working right so far except one piece.
In the third mission the players have to go in and stop my custom villain from destroying a standard character.
Since the standard character is Ghost Widow, and this mission happens in the Isles, I used Arachnos as my map's villain group.
Every time I test the mission, however, it ends in completion as soon as anyone steps onto the map. Don't have to go anywhere.
I've tried this in half a dozen ways now, and I've used all the testing tools, and it coems out the same every time.
I don't know what's wrong.
I have a second 'rescue the captive' mission later on, and that one works just fine....