For this achievement I got it in chapter 3: Beneath the Ice which can be picked from the custom game menu if you have the chapter unlocked. All you have to do is at the start of round 1 go to the center room with all the statues and on the right side, you will see a battery like the ones you fill up for pack a punch except this one won't have any color on the top. With this battery, you have to hit it 3 times with your melee attack. Once you've done that it will start to spark. Now that it's glowing continue to round 2 and leave 1 zombie alive and hold X on the battery. Holding X on the battery will change your vision which your supposed to use to find the runes for the easter egg but for this achievement just run around until you see him. My strategy is run back to spawn then run back to the center room and repeat until you see him. This may take a bit for him to appear but when he does appear your screen will rumble and temporarily slow you down but you'll see the ghost-like image of Klaus just shoot him a few times and you'll get the achievement. If you haven't seen him by the time your special vision runs out don't worry you can keep getting that vision back if you are on an EVEN
numbered round. The vision will not work on an odd-numbered round.
NOTE: There's seems to be a glitch to where if you find Klaus and shoot him the achievement won't pop. Just warning people about this. Will update the guide if someone finds a workaround or if it gets patched.
Here's a YouTube video for anybody who would rather have a visual guide