Command & Conquer: Red Alert 3

Okay, not too hard of an achievement.

First set up the following game:

Skirmish (Offline)

Game Settings:
AI Players: one
Map: Industrial Strength
Initial Resources: 40000
Random Bonus Crates: Yes or No (doesn't really matter)
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Player Settings:
Your Faction: Allies

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Enemy Faction: Allies
Skill: Easy
AI: Warren

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As soon as the game starts build plenty of Guardian Tanks and some IFV's. Roll in and destroy the enemy base except for any Ore Refineries and Barracks.

Done?

Now, start building up your money and several airfields. Once you've done this begin building up your base to allow use of Cyrocopters and begin building them, and while you're at it, start training a Tanya unit.

Done?

Once you have roughly 30 Cyrocopters roll them into the enemy base and start freezing any units spawning from the barracks.

Done?

When you have roughly 30-40 enemy units on screen, go ahead and save your game just to make sure you can retry if need be. After this go ahead and send in Tanya she should have no problem taking out so many units in this short amount of time.

*boop*
Achievement unlocked.

The above solution didn't work for me at all. I found the best way to do this was to start up a game against an easy Rising Sun enemy. The reason for this opposed to choosing Warren (Allies) is that they do not have rocket infantry or flack troopers which make going against Allies or Soviets pretty challenging. I personally chose Shinzo as my opponent, I think he's more inclined to man spam on easy and that's what you're going to need. Choose Allies as your faction because Tanya is a good compromise between Natasha (best commando option but no unit that can paralyze enemies) and Yuriko (worst commando option).

Build a reactor, reactor, refinery, refinery, barracks (+5 peace keepers &. 1 engineer), heightened clearance, reactor, armory (+10 guardian tanks), max clearance, reactor, airbase (+25 cryo copters and an Apollo fighter, I found it useful to build 1 to take care of the dragon flies. Build Tanya, keep her inside your base while you spam cryo copters. Send your guardian tanks out to destroy the enemy mech base and MCV, and any defences. Send your cryo copters out to freeze enemies. It doesn't matter where, Tanya can cross this small map pretty quickly. Once you have around 20 frozen (you should find the enemy has more individuals floating around the map) send Tanya in for an attack move (double tap ) in the direction of the frozen troops. If she takes damage just use her special ability to take her back in time and restore her health. I got this first go with this method.