You can get this achievement in any of the versus modes however I would recommend slayers as you can do a 1V1 with either a friend or a random. (With a friend simply take turns killing each other with physical attacks until you both get the achievement). You will need to kill other players 50 times with melee attacks to get the achievement.
I would recommend the character S.T.A.R.S Wesker as you can use the Hydra shotgun and L.Hawk Magnum for quick kills. When your opposing character has their hand on their hip they are low on health and close to dying and then when they are staggering and walking slowly they are in the dying stage when you can press X to melee kill them. You should get this achievement naturally during all the versus matches you will play for the other achievements however this will be useful if you need to artificially trigger it.
Hope this guide helps you get the achievement! :]