Observe where the red brotherhood npcs are spawning from a distance. They should be aggro’d by the town npcs. When they are aggro’d, pull the boss back to the fence you had to break down. You should be out of range of adds spawning in.
After you kill the boss, wait until the red brotherhood is aggro’d again by the town npcs and run past. A stealth one will spawn behind you once you get to the underhalls door.