there is two ways you can do this achievement
1.) put mutation on the four swordsman
2.) put mutation on chainsaw massacre
REMEMBER do this on easy
remember whatever mode you choose DO NOT SHOOT YOUR GUN WHEN YOU ARE DOWN!!!!!
and when your health is below 30 percent STOP randomly attacking your melee weapon because even when you shoot once you have to and i repeat HAVE to restart the campaign when you shoot you pistol because even if you die and restart the achievement will not pop so you have to restart the whole campaign
and something i noticed while i was playing with a random is that when your partner shoots dont worry because THEY will not get the achievement but only you will get it if you have never fired a shot
and if you are playing on the four swordsman mutation play it with people because sometimes the AI likes to look at you slowly being killed by a smoker even when they are right there
this can be on any campaign
and good luck because this is can be do but its more annoying then fun