I started using the method above also. Killed the first Knight using only grenades. But I remembered seeing in other threads that grenade kills in upward facing vertical combat counted too. So in the vertical combat section right after that first Knight I started throwing grenades, sometimes killing more than one with one grenade. The achievement unlocked during my first run on the vertical combat section. Could it really be this simple?
You can kill anything with the grenade, it just has to be at a higher elevation than you to count?