OK you know at the begging of the level where it tells you to unlock the gate thing in front and to the left of your base? well that is the whole point of the mission. there are 6 or 8 of those locks the you must well, unlock. so what you do is, go around that HUGE circle in the middle and look for the pillars similar to the first one you unlock. BUT the only way to unlock them is to remove all flood or covenant around. )You will notice that there are flood or covenant each one) once you unlock all of them yo will complete the mission. if this still doesn't help you, my Gamertag is TrueSoldier 00. i'll be glad to help you out. and i can also get you the achievements for each level