Hi friends, i have finished halo5 on legendary (solo)
But the achivment has not been unlocked
In the game it Says i have finished all the missions on legendary but in the achivments, it says i have done 73%
What should i do?
I dont want to redo 4 missions on legendary 
If at any time you started a mission in co op campaign and not solo, even if by yourself it probably counted as co op,
That looks to be your problem.