It does exploit the regular/seasonal sales player high-tides (like a surfer using the moon for better waves), however; and the occasional wave of new (and sometimes old) players that come(/return). If at least to obtain the Mercenary Mode achievements. Or make an attempt, at least. This post should help improve the chances of obtaining it for everyone (myself included), and who knows, even revive the multiplayer modes/activity, if albeit briefly (every EDF:IR sale? Regular enough for everyone, New and Old?).

If this works well enough, feel free to apply this method to other games (and sticky the post) that can use a MP-boost via this method (especially if they have similar BS achievements to acquire that requires (multiple players simultaneously) being online in an otherwise (un)dead game. What's more regularly occurring than (seasonal) sales? Though, expect diminishing returns on new players annually/seasonally.

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Fix the game before doing anything, including playing the game, itself, proper. I know "It's literally unplayable!", that why I brought you the link to fixing it. It's well worth the effort; I actually had fun playing this game. Enjoy.

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3092809280
EDFix GFX Patch:
https://community.pcgamingwiki.com/files/file/1882-edfix-earth-defense-force-iron-rain-fix/

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Simply put: I want to get my (impossible) achievement (1st Time Merc, and Ensign Rank; Online achievements can be done solo; except Mercenary-based ones), just about as much as everyone else. To do that, we need players (experience doesn't matter (Stormtrooper accuracy is even allowed, no standards. I mean, just check out the AI allies' accuracy. Dark Helmet would be proud.), even if Mercenary Mode was your first option.); and given that sales times are an excellent opportunity for new players to enter the fray, why not help new and old players alike out with an otherwise BS achievement to obtain?

Simple request:
I'll be doing my part (at least during SALE days) to get the ball rolling. As I finish up collecting my remaining achievements (just collecting remaining loot, by now), I may open a new room to get a game going; but anyone else with a copy (New and old players, alike), I recommend making at least an attempt to join a room, and if no room exists, just open a room up for even 15 minutes. It'll be a dice-roll to get someone's attention; but given the game will be on sale aplenty, and EDF6 may be a little much for some, still; curious eyes will check this game out.

So yeah. Feel free to use a stupid name for the room like "Ensign-Up, Mercenary!" (Feel free to borrow.) or something else equally stupid to draw up some attention. I think it only takes a round of fighting to get Ensign Rank; but it wouldn't hurt to play for 3-5 rounds to make sure it sticks, and give some time for more multiplayer activity to generate. It's worth a shot. Good times to attempt Multiplayer Mercenary Modes would be during the beginning and end of your playing sessions (just grinding for those guns and gears, or Disaster Mode beatability). Just keep a room open for 15 minutes or so, and leave if nothing happens. There should be plenty of players available at any time to make their own attempts. so 15 minutes of waiting on someone should be generous enough. I think.

Feel free to discuss other ways of getting this achievement, offline if possible (Savegame Edit, maybe? Sounds risky. What with any Anti-Cheat. Attempt at your own discretion. Caution: Savegame files are in Japanese. Have fun!).

For New Players:
Don't worry about entering Mercenary Mode, only to get rekt alot.
(...You'll get your chance. You die, you can upgrade your health (LVL+1), weapon upgrade (More likely just a random weapon, better than starter weapon; GREEN DROIDS also give you one while you're still alive), or get an item (Random offer, as well). REMEMBER: With Drones, Green Gives Guns/Grenades, Yellow Takes Gems.)

For Veteran Players:
You WILL start with Class-E equipment. Sorry, but your crap remains in the Base Game (Off/Online). Mercenary Mode is separate from Base Game Online Mode.
(...But give yourself time, you'll get your better crap back, and maybe a demo of better gear that lies out of reach, for now, too. Take advantage of that, and see what you can bring back to Singleplayer Mode ((Weapon) Experience-Wise).)

For Mother-EDF'ers who beat it on Disaster Mode (beat game to unlock), grinding away: (And who knows? Got the Complete Arsenal Achievement. Good work! Wasn't that fun?)
Yes, that means your Class-AA equipment is not invited either. So no stabbing everyone with the Sinister Arcsaber for fun, or vaporizing everyone with Curtain Calls and Linked Super Blasters.. So, no (Post-)Endgame gear; only N00b gear, for the time being.
(...Just hope that you get something you are REALLY GOOD with (Choose PA-Gear wisely), and have a good enough battlefield condition to get your points in. However, keep in mind, if it was OP in Single-Player, you, too, are a fellow Bug just as easy to kill as the rest out there.)

(TL;DR: On SALES DAYS, Out of courtesy, please take it easy on the N00bs. For now (Limit mainly to days that EDF:IR is ON SALE). Gotta re-gain interest in Merc-Mode, at least. Being a griefer/jerk doesn't help anyone.)

Assuming Multiplayer Online/Mercenary Mode gets a revival (regular MP activity back up to like 5+ (1-2 more likely) rooms active on (daily) average, or something); please resist being jerks during the infancy or revival for at least during SALES DAYS, if general interest is returning. Once it's back in full player activity (5-10+ rooms daily average, let's say), then furthermore resist being bigger jerks and start hacking away and N00b-hunting. Give enough people a chance to reach Ensign Rank, before the inevitable HAXX and such come along to kill multiplayer for everyone. At least, lighten up on the BS behaviors for at least during SALES DAYS. That's the least I request, for New Players'/Achievement-Hunters' sake. Other than that, go nuts; words can only do so much for so long.

Ensign Rank Requirement: Win 1 Match of Mercenary. That is all.
If you lose a match, then you don't get the achievement. I know, because I lost a match; but didn't win any yet. I would assume my opponent got their achievement after beating me (feel free to comment for verification; I can still win a match and verify for myself, otherwise).

MODE: Scavenger
3 Rounds shooting all kinds of critters, and giving them to (Yellow Ring) Drones across the playing field
MAP: EDF Base (Day)
GAMEPLAY:
1- Shoot the bugs (and other big guys)
2- Collect the Gems (1 Gem is acceptable, 3+ Gems you rock!)
3- Deliver the Gems to (YELLOW) Droids.

MODE: TBD (???)
(ONLY SCAVENGER MODE CAN BE ACCESSED IN MERCENARY MODE) (For now?)

Given the Room Options, I'm guessing more modes, besides Scavenger, can be unlocked the higher the rank you reach? I dunno. Might have to actually take a gander at the instruction manual for this.

-It's Multiplayer. Expect shenanigans. Watch your (Friendly) fire.

-When you die, you are given an upgrade (Pick 1 of 3)
1= HP-UP (HP LVL+1)
2= Weapon Upgrade (Weapon 1 OR 2 gets upgraded; most likely your crappiest weapon gets the upgrade, for balance reasons.)
3= Item Added to Inventory (From Air-Raids to Grenades to Vehicles; random items offered.)

-When you die, you also drop ALL YOUR LOOT (Gems). I had many gems when I died, and dropped a huge Gem worth the sum of what I collected just earlier.

-YELLOW DROIDS are where you deliver your Gems.
-GREEN DROIDS are where you get Bonus Loot from (Option 2 or 3 from above).

-Choose your PA-Gear wisely:
=Trooper (Ranger) can carry many things (Option 3 loot)
=Striker (Fencer) can dual-weld (2x the damage, 2x the fun) (Option 2 pays off)
=Prowl and Jet-Lifter (Wing Diver) own the skies (Option 1 may serve better)

-[T]alk, if someone is AFK (and be patient). Especially if they're not (READY) to play.
-If Host: (READY) yourself, and be patient. If you HOST it, they will play.
-If Guest: [T]alk to the host, especially if they come out of AFK.

General suggestion:
If you already gained the achievement, host a session, and help others obtain it, too. When the game is on sale is a great time to setup camp. If you're lucky, others have the same idea, and setup rooms too. On that note, however...

Assume that NOBODY GOT THE ACHIEVEMENT (OF ENSIGN RANK, in this case, applies to other games, too), INCLUDING HOST OF ROOM (and not just me (and EDF:IR)). Play for at least 2-3 rounds (especially if host is no longer AFK; it doesn't hurt to [T]alk to them, either; just in case.). You got your achievement, help the other guy get their achievement, too. Help the (virus) of achievement spread, and make this achievement "Rare" no more.

Now, I'll still fight fairly, and earn my achievement properly (I have the skills/experience/persistence to back it up); and by the time I DO get my achievement, I'll still leave a room open for anyone else to earn theirs, while I AFK (if the case, when "Leaving a door open", like last hit). To whom it may concern, you're welcome. Feel free to leave a room open, and help others out, as I have helped you out.