To get this achievement, you have to beat Mission 4, Infinity, on either Easy, Normal, Heroic or Legendary difficulty.
Here are some tips for the mission Infinity:
~ The first part of the mission isn't too bad. Once you find some marines, you will have to defend the building for a little while. There will be several Knights. There are three turrets in the area, so make sure to utilize them well. Don't use up this ammo on the Crawlers, save them for the Grunts.
~ After that, there is one more hard part in the mission. You will come to a point where there are several more Knights. Pick up the Rail Gun, and choose your shots wisely. Once you kill one, pick of their weapon or get some rail gun ammo from them.
~ The last part of the mission is with the Mantis, and I didn't find it to be very difficult.
Here are some general tips if you are having trouble beating a level:
~ 1. Know how to kill the enemy the most effective way
Grunts: Best option is to shoot in the head with a DMR, BR, Magnum or Light Rifle
Jackals: Use a long range weapon, and shoot where their hand is holding the shield. If you are up close, I recommend using a assault rifle and just spray the enemy, but just be careful not to waste ammo.
Hunters: Best choice for a hunter is to either stick them with a sticky, shoot them with a shotgun in the back, or fuel rod/rocket launcher them. It will take several fuel rod or rocket launcher shots to kill them.
Elites - If you have the plasma pistol, charge it up and shoot it at an elite. This will take away its shield, and you can just headshot them with a DMR or BR. If you don't have a plasam pistol, you can use a needler or headshot them with a DMR or BR.
Promethean Crawlers - Crawlers aren't too bad to take out, just try and headshot them with any long range weapon. They sometimes come in packs, so be sure to dispose of them quickly.
Promethean Watchers - Promethean watchers can revive knights. Be sure to dispose of them quickly, as you do not want to have to take out knights again and again. Any long range weapon will suffice.
Promethean Knights - Promethean Knights are fairly difficult to take down. You can really take them down with a lot of weapons, but I recommend the rail gun, or the assault rifle. You can also take them out with a turret effectively and the suppressor. Once their blue shield goes down, keep firing until they perish. Knights are the one of trickiest enemies in the game.
~ 2. Don't waste ammo!
Ammo is scarce on the battlefield. Make sure not to burn up two clips for one Promethean Crawler. Use your ammo wisely, and always know where you can get ammo if you run out. Also, don't use weapons like the rail gun on Promethean Crawlers, save it for the Knights.
~ 3. Use your abilities in the campaign
The best of the new armor abilities in my opinion is the Auto Sentry. The Auto Sentry is great because it provides a great distraction, and also can take out enemy's. All the other armor abilities I don't find to be as effective as the Auto Sentry. I found the Active Camo, Promethean Vision and the Hologram to be pretty ineffective, except Promethean Vision helped in mission 4. The Hardlight Shield is pretty ineffective, only works a bit on Mission 2.
I will post these tips for each of the mission achievements.
Here is a video on Mission 4: Infinity
*Video Credit to VesperAzure