The other day I was clutching the end of a wave on Giant and was down to one banshee at full strength chasing me through the control room at the top. I grabbed some ammo in the room below and ran down the ramp where a few swarmers were hanging out. I took one shot at a swarmer and somehow the banshee died and showed up in the kill feed instead. I wasn't using any dot ammo or powers either. It was like the game registered the swarmer death to the banshee. I didn't complain, but it was strange.
That Banshee was seen on 4th Ave. with a "Dis b bullshiet!" sign.
I once gave a banshee a cold stare and she jumped off the LZ on glacier and committed sudoku. Does that count? 
If it's true, murder by suicide is amongst the highest of badass kill medals.
One time I was in the LZ on Ghost and a pug was running past the ammo box and into the little house there. I Krogan heavy melee'd a Possessed Abomination up and out of the LZ, right through the door, and blew up the pug
this does count
By your logic, this player was a boss (or BAWSS). If that's the case, tally it on up!