Enslaved: Odyssey to the West

First, purchase the plasma penetration upgrade and at least the plasma damage 1 upgrade. Then when 3 enemies are lined up fire the plasma blast at them. If it goes through all three and kills them you will get the achievement.

As shown in the video below, one place to try is in chapter 6 near the end when all the mech keep rushing you in narrow ground.

If (like me) you didn't upgrade the staff until late in the game, you can get this achievement reasonably early on in Chapter 14.

I got this in Chapter 14 once I had the Plasma Penetration upgrade, both Plasma Damage upgrades, and the Stun Effect upgrade.

Once the level starts, there are some leeches you need to kill and two mechs, so deal with them. Following this, there is a brief platforming sequence, followed by some more leeches, followed by some more platforming, then you will come to a third set of leeches.

As soon as you spot this third set of leeches, three mechs will appear. Shoot the leeches quickly (one in front and two behind you), then try and cluster the mechs into a small block or line (corners/walls work well) and stun them with a stun shot from your staff. Stunning them allows you to run around and find a good angle to shoot all 3 from.

If you can't get a good angle, just wait for the stun to wear off and have another go. If you mess your shot up, just reload the last checkpoint and have another go, it's not very far into the chapter so it doesn't take long to get there.

Enslaved: Odyssey To The West
Out cold & KaBabed Achievement Guide
Chapter 8

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I did this in chapter 5 on the bridge (there is a nice situation with 3 attacking mechs) but i think you can do this at any time when you're attacked by 3 or more enemies.
Stun 3 of them and try to find the right angle to shoot them all with one plasma shot. Try this after you purchased the plasma-stuff-upgrades.

I got this in Chapter 9 once I had the Plasma Penetration upgrade, a Plasma Damage upgrade, and the Stun Effect upgrade

when you get to the section where you find the boat take out the mech that fires at you then look near the boat and you will see 3 mechs in a row get near so they are in a line in front of you and quickly hit them with stun ammo line your shot up and hit with a plasma blast

i did this one in chapter 4. theres an auditorium area you get to with 3 mechs and a shocker mech giving you 4 to try it on. i bought the plasma penetration and the plasma damage 1. i weakened the 3 mechs " for good measure " and stun them and i took the shot. also i hit the shocker mech and 2 of the other mechs and got it and didn't have to weaken him.