Bros to the Close
To obtain this achievement you must have all 6 marines you meet up with survive until you open the door in mission Infinity. (on a hill with 3 turrets and barricades) the most difficult part of this achievement is getting to the hill with all 6 marines still alive.
When you do the mission Infinity you will meet up with 2 marines at the beginning of Rally Point bravo (yes you can load up rally point Bravo to start this achievement immediately instead of starting at the beginning of the mission).
The first thing while doing this is play Heroic since that will make the enemies the easiest and the marines more likely to survive. Also if you can do this with 3 other friends (with experience playing halo 4) that will greatly increase your chances to obtaining this achievement. If you do this solo on Heroic its still possible but will require a good amount of skill and knowing where enemies spawn.
In the first area you must go through a open door, there will be a good amount of crawlers, 1 knight 1 watcher, you want to take out the watcher when you see it since he can revive the knight. Right when the door opens there is a DMR you should grab over your assault rifle on the barricade in front of you. Use your pistol to take out the crawlers and grenade/DMR the knight.
Going up the hill there will be a group of crawlers and another knight. Killing them with the same strategy. Then when you meet up with the last 3 marines you want to kill all the crawlers with the last of your pistol ammo if any, or a precision weapon. Dont forget to grab the rail gun against the barrier to shoot the knights with and DMR the rest of their health.
Then when you are running up the hill you should activate the next checkpoint. If you still have 6 marines you are almost done with the hardest part. Take out the 2 watchers that start building the turrets first with any precision weapon you have, take the knight out next, and take care of the crawlers, the final test is in front of the door you need to activate. There will be 1 more knight, use the rail gun and a DMR (don't forget the area has DMR's laying around so grab one if you see it, also a rail gun next to the left turret).
Once you activate the turret you are at the easier part, if you have 6 marines you should have this done now, easily with 4 people, if your doing this solo theres a good way to do this. Right when you activate the door run down the hill and kill 1-2 crawlers and turn around looking for spawning knights. Melee them in the back right as they spawn (they spawn in the same spots every time) theres a total of 5-6 knights at this point, keep meleeing their backs as they spawn, and help kill some of the crawlers which most of the marines will take care of. There will be 1 spotter at this point, make sure you kill him immediately.
Once you activate the door the achievement will unlock if all 6 marines are alive. Again doing this with 4 people will make this much easier, doing solo is do-able but will be really annoying if you haven't done this mission often alone. Even then it can be a challenge.