Okay, I discovered this today by accident. As far as I know, it only works for heroics (I did it for Sethekk) and these are only the steps that *I* took. It may be much simpler, but this is a 100% way to recreate it.
1) Have a friend who is saved to a heroic (without the boss you want to kill down)
2) Enter the heroic by yourself and kill the boss(es) you need
3) Leave the instance, have the friend invite you to a group (I was invited to a raid and I don't think it makes a difference but if a regular party doesn't work, try a raid)
4) Re-enter the instance, kill the boss(es) you need
TA DA