A nice idea, but I see it being awarded one of two ways:
Waiting for monsters to crowd in the middle of a shallow barrels level and setting off a chain reaction, or
with the BFG9000, the only non-nuke weapon that could plausibly do it without help.
There aren't many things that do enough damage over a big enough area that you get at the right time for those numbers. Really, this medal cries out for masterfully crafted BFG shots. Menacing with spikes of shotgun shell and encircled with bands of red armor.
Also, is this one shot, or one attack? It seems possible, though unlikely and sort of wasteful, to overcharge a sniper-modded plasma rifle in a long hallway full of 18 formers... if that's even possible in anyone's lifetime.
Maybe instead of 'one shot' it could be a sort of chain system, at least one kill each x seconds/turns, though then of course there's the problem of certain builds getting it on accident on a normal run.