to add to cooper101183, for each nightmare campaign you need to finish mission.
do all objectives. these are long missions.
for me after mission was complete I had to hit 'X' to go to main menu to get achieve to pop. choosing the to battlefield did not unlock achievement (happened for mission 1 when I did this). if that happens, only way I got to pop is starting new character and doing that mission again. may have been rare glitch, but just aware
to make easier to beat u can use characters u already have, so just beef them up in other mode if u have any trouble.
forming armies (rt + a when 2+ leaders or at deploy stage w/ y) makes easier to beat as well.