there simply isnt enough rebels on a campaign to do this, and since it only counts the rebels of a single "session" you cant just replay the game to get the rest, others say to use the firing range, but i couldnt get this to work, try it for yourself and see fi it will work for you, but a simple solution is that after the chapter low there arent anymore rebels, so use the command console and input "give npc_citizen" without the "", then kill the npc and spawn the rest until you get the achievement.