I was lucky to get mine. They way I did it, I melee'd a guy that probably had bio boost on...and when he tried to melee me back, I was running away from him and hit him in the back with the grenade. I think he had taken a bullet or two after I hit him...so looking at it like this:
With bio-boost, let's say he had 125% health. I melee'd him - down to 35%...then he took a bullet or two ...down to 1%...the grenade finished him off without exploding. I think for someone to die from a grenade impact, they need to be on their ABSOLUTE last % of health and you need to catch them at the exact right moment since health auto-recovers. Pretty rare. Only ever done it once!