I'm definitely not against boosting achievements, but for this one I really would suggest doing it legit. I tried boosting and got nothing, but after a good couple of hours over the course of two days I had snagged the achievement. Here are some tips to help you...
1) It's called GUARDian
Most of the time when you join a match players will rush the enemy leader, hoping to end spawns for the opposite team. Now this certainly isn't a bad strategy, but sometimes when the whole team is attacking, the leader can be left alone and hopelessly outnumbered, causing your once perfect strategy to become mush once your infinite spawns are taken away.
So, stalk your leader. Use left bumper to see where he is located (he'll have either a blue or a red star over his head, depending on if you are playing as Locust or Cogs) and make sure you know where he is at all times. Even having one extra set of eyes looking out for badguys can really help your leader stay alive longer, giving your attacking team more time to take down their target without having the stress of one life over their head.
2) Smokes aren't as useless as you thought
Why do they even give you smokes? I've often wondered this as I searched for some frags or ink. But most players grabbed these precious grenades before me, often leaving me with the stinky smoke. But was it really that poor of a tool?
Frags and ink are used to disable enemy forces from approaching, usually placed behind a camping spot so that you can shoot oncoming troops without having to worry about bullets in your butt.
While smokes don't possess the same brute power as their cousins, they can still be used for the exact same purpose. Place them behind you. Try planting them on walls just around corners so that your enemies don't notice them and are taken by surprise.
Smokes make a distinctive sound when they are triggered. As soon as you hear that sound, stop what you are doing and turn your shotgun towards your rear.
Also, because most people regard smokes as useless as I once did, make sure to grab plenty. You start out with one every single time you spawn, and often players will trade smokes for frags or ink, which you can easily pick up after them.
3) Power Weapons are your friend
You really need to learn your maps and remember where weapons spawn. It will give you a distinct advantage if you can grab a sniper, torque bow, or mortars.
Furthermore, you need to practice which each of these weapons. Get acquainted with their reload times, recoil, and other traits specific to them.
4) The Basic One-Two Punch
If you can't find power weapons, you always start with both a short range weapon and a long ranged weapon - your lancer, and your shotgun. Spray enemies from afar, and as they get closer switch to your shotgun.
Both the lancer and the shotgun are used very commonly in Guardian. Whenever a player dies, try to pick up their ammo if you can. Shotgun shells can often run out especially fast.
Also, if you have a power weapon that is running low on ammo it may be a good idea to find a basic weapon with plenty of shots rather than hoping your last couple of sniper rounds will hit.
So, thus are my tips. I hope you find them useful, and good luck!