Just wanted to chime in that I think Vanguard is in a pretty good place right now. I get killed by it a lot, but I also kill a lot with it. I find it is easier to use effectively on certain maps (Crait, Maz's Castle, Tatooine) and in certain phases of the battles.
The original poster mentioned the most important part of getting this milestone: Flanking. You can often get behind or to the side of a group of enemies by flanking and take out 2 or 3 with relative ease.
Crait is a great map to get this on (either attack or defense). Running through the tunnels with Vanguard on will give you lots of opportunities to one shot guys, and every now and then 3 enemies will be packed together.
