In Numa Numa Trail you'll come to a section where the characters start to talk about an enemy sniper.
Eventually you'll drop off a small ledge and there will be a tractor that you can take cover behind and your team will smoke the bridge and tell you to rush across, immediately after running across the bridge take cover behind the rock on the left side then once you're ready activate Wade's focus ability (D-PAD UP) which will slow down time and will highlight the enemy sniper in the tree in the distance, simply take out the sniper without getting hit and the Achievement will be yours.
If you get hit or die by the sniper quickly pause reload last checkpoint and you will be able to attempt it again. I personally died to the sniper and quickly reloaded my last checkpoint and I still got the Achievement.
Here's my Achievement Guide for those who prefer videos.