This achievement will pop if you get all your kills in one sitting.
After every game let it finish and bring you back to the main menu so it registers and you’ll see your tracker go up. If you quit the game your progress will reset but your tracker bar won’t and you won’t see any progress until you get more than you had before the reset.
So basically get your 50 kills without totally quitting out of the game.
I used damage and support heroes and the tracker went up at the end of each game at the main menu