I would like to add an addendum to the previous solution already created by FxH Final as the solution itself is fine, but I think needs some details, primarily for those who are just starting off or are coming back after a LONG hiatus.
First and foremost, you're going to need to complete the "The Archwing" quest. Once complete, you need to obtain the Landing Craft Foundry Segment. This can be purchased from a clan Dojo for 15,000 credits or for 175 platinum on the Market. If crafted (purchased for credits at a Dojo), it will require the following components:
* 100,000 credits
* 50 Grokdrul
* 50 Iradite
* 1200 Oxium
Grokdrul and Iradite are resources unique to the Plains and can be found nowhere else except there. I would advise using a resource booster if you have one to double your resource collection intake.
Once constructed, you will be able to construct the 50x Archwing Launcher. You get this for free from the Lotus in a message. To build it, you require:
* 50 Iradite
* 50 Grokdrul
* 30 Fish Oil
* 600 Circuits
* 7500 credits
You get Fish Oil from fish that you catch and take to the Cetus fishmonger's Fishing Services. All fish give scales and oil. A few large fish will get you what you need.
Once constructed, you need a loadout to hunt this specific achievement. The thing is with this Archwing, it does not have your Archwing weapons, you will only have access to the weapons you have on you and your abilities of your Archwing. For my primary, I used that free Vectus sniper that Xbox gave out. If you have a strong sniper/long range weapon that doesn't lose a ton of damage fall-off, use that. Some suggested mods that I personally used were, Vital Sense, Split Chamber, Serration lv. 4, Charged Chamber lv. 3, Hush lv. 3, and Metal Auger. If you have others that are better, use those. But I still highly suggest a maxed Hush as it will make your weapon silent, making it easier to pick off enemies from afar. Make sure to equip your Archwing in the Armory before heading out.
Now, load up the Plains. If you're grinding this out, I suggest playing on solo or friends only as you won't get randoms that try and summon you back to Cetus before you're ready. Summon your Archwing by looking right at your feet a few meters outside of the gates (press
on your controller and select where it is on your wheel with
, and press
). Once summoned, look immediately up and get some air by pressing in Sprint for a second or two and press it again. Now you're comfortably in the air.
Get used to the controls before you go off sniping enemies so you don't panic-smash yourself into something as you flee to get some comfortable space between you and your targets. One large note is that you don't have that large amount of health when your Archwing is equipped. Meaning, that you'll fall out of the sky well before your health or shields get massively damaged. To my knowledge, there is no way to strengthen them to last longer, hence this sniper, take it slow, tactic.
Personally, even with taking it slow, and with 78 total rounds of sniper ammo, even stopping to resupply once, I only used around 3-4 total Archwing summons. So if you get 25+ kills per summoning, you're golden and won't have to worry about running out of summons anytime soon before you finish the achievement. Just take it easy, line up your shots, and scootch a little out of the way when enemies do discover you and start to shoot at you. The only thing you really need to beware of are Bombards, MG turrets, and rocket turrets. The rest you can dodge just by moving slightly left or right.