XxDeadShadowsxX

So I beat the Campaign on Legendary by my self and I didn't get the achievement for it..or the achievment for the last level. The game sease I beat all the levels on legendary, Wich I did. The achievment to beat the game alone on legendary is at 67 percent...Wtf. The game screwed me over big time.. Help?

So I just beat Halo 4 on legendary by my self on the MCC and guess what? The achivment is at 67 Percent....Wtf? Im about to just return this game. I absolutly love halo but seance day one this one game has been screwing me over. Im at 97% for beating all of the games on Easy and Normal..Not to sure how, I already beat those. Is there a way I can get the achivment without having to do it all over again? It's really annoying and just Grrr ...