You can achieve this through Local Play in the Multiplayer menu.
Either use a second controller with another profile/Guest on console, or go solo against a Bot.
3 maps come to mind to get this done quick and efficiently.

In the video above I display the map "The Abandoned Base" which has the Pentagram of Protection spawn outside (you will have a water entrance, a window at the top (only reachable with a double rocket jump *Grenade and Rocket combined in a jump* along with the 3 lifts on the right side.
Once you nab the Pentagram of Protection just jump into that water and go for a swim; pick up the LG that spawns under-water and await another enemy to drop into the water with you.

Fire the LG and receive the achievement.

Note that firing the weapon under-water with the Pentagram of Protection will sap all of the Ammo with one click in comparison to using the weapon in normal combat. You will see this in the video above as I need to fetch more ammo to get the frag again.

Note that the Pentagram of Protection, water and the LG is the only method of completing this achievement.
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Once you start you should find your way to the lower levels where a few rooms are filled with water. There are three different ways to access these areas and are not hard to get to.

Drop through a hole in a room above (it has three columns with nearby armor), drop into a pit down stairs past the aforementioned hole, or walk down submerged stairs under the walkway with a button.

The room of importance is easy to find, it's a large room mostly filled with water with three columns. Within the room there is a ledge/area in a corner that you can jump onto out of the water.

In this area you can find a Thunderbolt gun, and a Pentagram of Protection. You'll want to grab both.
Have your second profile/player drop into the water/wait for a bot/player/Secondary Profile to enter the water, then kill them with the Thunderbolt - Click - Achievement - Success.