Warframe

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.

There are 2 ways to obtain this achievement. Remember that, in both cases, it is cumulative and you don't need to do that in 1 run.

FIRST WAY
You can simply pick up Titania, use the 4th ability and kill enemies while flying. Clearly, this is not the way how the achievement was intended, but it works.

SECOND WAY
This is the standard approach. You have to build the Archwing Launcher Segment in the Tenno lab of your Dojo or buy it for platinum in the Market. After that, you have to install it in your Foundry and build the Archwing Launcher. Equip it in the Gear menu and use it on the plains to call in the Archwing for you and your mates. Remember that, in this case, you are still using the standard weapons, not the Archwing ones.

BUG
All the achievements related to the Plains can get glitched randomly. Digital Extremes is aware of these glitches and they will get a patch in the next future.

I just want to quickly mention some things that you can do to earn this achievement that you might not be aware of as of February 2022.

First, you'll notice the achievement description say "Kill 100 grounded enemies while in Archwing Mode above the Plains of Eidolon". However, if you press Start, go to your Profile, then to the Achievement tab, it says "Kill 100 grounded enemies while in Archwing Mode above the Plains of Eidolon, Orb Vallis or Cambion Drift". So, you can do it in any of the free roam areas.

Secondly, you don't even have to have an Archwing. You can either:

A) Hijack a Dargyn somewhere in the Plains of Eidolon and get kills while in that. This is kinda tricky to do, and they're pretty fragile, but does count towards it.
B) Get kills on a K-Drive. Yes! I was surprised myself. I got a K-Drive waaayyy before an Archwing. The downside here is that you can only use your Secondary weapon. So, when you run out of ammo, you have to go back to the base, complete the "mission", then head back out.

I hope this helps!