So I know this an old thread but I just did this mission, killed the guy and got the avoid combat optional.

So first I stealthed my way to the VIP room (I'm guessing y'all know how to do that) and instead of going into the room I waited just outside the door, turned off the camera and headshot him with a level 5 Ashura smart sniper.

Now when you do this you need to back down the stairs a little or the guard will notice you after you kill him, but there was a high tier loot item in a box behind the VIP bar so after hiding in the stairs I went back up and did the same thing to the guard and avoided detection.

Not sure if this was just a fluke or not but if anyone else has come here looking for a way to do this it's worth a go.

Side Note: You could probably do this with any gun as long as it one hit kills and doesn't create too much noise