There is this achievement called “Bros to the Close” in Halo 4. The achievement requires you to protect your fellows in Mission 4, and you can let anyone die. You need to do it on Heroic or higher difficulty level. How? Here I am to help you out.
Achievement: Bros to the Close 20
Completed mission 4 without one preventable Marine death on Heroic or harder.
You will find a total of 6 UNSC soldier you need to protect during the mission. Two of them will accompany you after your meeting with commander Lesky, and other 4 will be at the base of the hill. Now, since they aren’t going to sit back and wait for you to finish off the enemies, you will have to play the additional hero part to save them.
The best way to do it is avoid rushing through the battlefield. Rely on long range weapons and pick enemies from distance. This will keep them safe. If they happen to be target by enemies, or you find them pinned down, leave everything and head over there to divert enemy fire from them.
Cruel indeed, but you will have to do it if you are looking forward to unlocking the achievement. If any of the marines die, you are always welcome to reload the checkpoint. Make sure that none of them has died until Cortana opens up the hatch, or you may have to play the mission from the start.