This issue has been mentioned multiple times and even after attempting the “death switch” with help from the other teams this achievement isnt unlocking. There is no visual tracker of how many wounds you have up either.
I got it recently after doing the death switch. My recommendation is just throwing the knife to the ground before you ult. After you ult, call your knife back and hopefully it hits some players. Takes a bit of luck but that got me after attempting that strategy about 3 games.