I know for a fact that this can be done. Don't know how though, 'cos I only played the mission which uses the same principles.
You had to sneak up on the boss with your weapon holstered(past his guards), then you killed him, and then the guards started chasing you.
For any second thoughts, It didn't work with the variable "boss killed", cos I tried it, took out the gun in front of the guards (with boss alive), waited a little and then they shot me dead.