Mass Effect Legendary Edition

Once you have access to the Silversun Strip area of the Citadel, you can access the Armax Arsenal Arena. What you need to do is defeat the Mirror Match enemy set, which is basically just a bunch of Shepherd clones with very high combat stats.

To do this the quick and dirty way:

1) Turn the game difficulty down to narrative and start playing arena matches. I personally found the geth to be easier than Cerberus. Play several rounds, aiming for a silver score. With three or four silver medals, you can unlock combat multipliers and more advanced enemies.

2) Fight the elite geth set with several multipliers active (I chose weak shields, no medkits, and no ammo crates--remember, we're still on narrative difficulty here.) This should get you a gold medal.

3) Gold medal in hand, trade it in for the Mirror Match enemy set. Now turn off any combat multipliers and start the match.

4) Once you've beaten the first two waves, thin out the third. When there are one or two Shepherd clones left, pause and raise the difficultly back to normal. Finish the round and the achievement should pop.

The above guide is perfectly sufficient to unlocking this achievement. However, if you are going for the Insanity achievement and don’t want to risk using different save files, then I suggest using the method @AuraofHeroism in the comments of the above guide used, as well as my own strategy, to win this achievement. This is the exact method I used, and I won in fewer than three attempts: The M-11 (the pistol Shepard acquires at the beginning of the Citadel DLC) is sufficient in and of itself to take out all the clones, without changing the difficulty. For armor mods I used a scope for distance and cranial trauma (the headshot modifier). I equipped no other weapons, so my cool down was +200%. I took EDI and Miranda with me because both have overload and incinerate. If Miranda isn’t available, Garrus and Kaidan are useful as well. If neither of them is available, then I would go with James for his Carnage ability or Liara for her Warp and Stasis. I used Wingman as my stage and turned on the damage bonus in the Armax menu. The only downside is the lack of ammo, but there are plenty of ammo boxes. This can be used by any class, simply due to the overpowered damage the M-11 provides. Each of the three waves spawns a total of 6 clones, regardless of the classes you’ve previously taken out. This contradicts what I’ve read in the Mass Effect Wiki, which stated that two of each class spawned in the first two rounds, with one of each spawning in the final round. The first round consists of the Engineer, Adept, and Soldier classes. I personally found the Engineer to be the most threatening, since it was the most aggressive. The Adept and Engineer both have the N7 Hurricane, which can obliterate your health in a short amount of time. The clones also spam like no other, with the biggest threats being the Engineer’s Incinerate, the Adept’s Warp (which can hit you while in cover), and the Soldier’s Carnage. The next round is filled with the hybrid classes: Infiltrator, Vanguard, and Sentinel. The Vanguard is absolutely the biggest threat, as it is easily the most aggressive and has both Charge and Nova. Charge gets the clone in close range, and Nova is an area-of-effect ground pound, both of which stagger you. The infiltrator is also tricky, since it has the tactical cloak and might be difficult to track down. The final round utilizes all character classes, of which the Vanguard is still the deadliest. Once three of any clone are taken out, no more clones will spawn. One last note: the clones almost exclusively target you, so squad survivability is a low priority. Good luck everyone.

*Edit: Changed gun to which I was referring to the M-11 (previously written as M-22)