Hey there MochiKing. Appreciate your report about this. If you could provide a link to a video of a recent game where you felt that you should have gotten the Massacre medal but didn't? It may help us in an investigation.
I can't say it's completely broken because I've managed to get 2 in the past week but I wonder if it's buggy. I have 78 Void Based Demolition medals (Nova Bomb) but only 11 Massacres since Gambit came out. I would like to know how many kills it takes to get Void Based Demolition because I see that after almost every Nova Bomb I throw but the Massacre just doesn't seem to happen even if I keep killing ads.