My situation is i completed the game with 30 rebels killed and i only had that. Once i completed it i changed the difficulty to good cop so in a new play through where i got the collectables it would be easier and after all that i went into the firing range and killed 50 rebels but did not get the achievement.

Simply switch the game mode back to the one you switched it from which in most cases for people it is the bad cop difficulty and then you should get it once re-killing 50 rebels in the firing range. (i don't know why but it does not count previous rebels from your original play through)https://steamuserimages-a.akamaihd.net/ugc/2294088243491127399/0A3707D2D0C0B31D6FD6134F0AB01ED16BA9D9CA/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false