To Battle!: Hell's Crusade

Run Away is mutually exclusive with Sneaky Bastard.

It is recommended to do Run Away achievement first, using your two scouts to rush to the northwest part of the map to discover the Behemoth while at the same time having the other scout units rush to the NE corner, so the moment the exit is available, you will run away.

Of course, there are demons about, try and outrun them.

After the achievement pops, exit to main menu, load your save and restart the scenario, this time killing demons and going for Sneaky Bastard.