Quake

You can achieve this through Local Play in the Multiplayer menu.
Either use a second controller with another profile/Guest (confirmed by commenter (AirNath)), or go solo against a low difficulty Bot (confirmed by commenter (C64 Mat)).

The only setting I changed was the Map to DM5: The Cistern.

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 to enter the water, then kill them with the Thunderbolt.

Please note, as a commenter (AirNath) below mentioned, and something I forgot to make clear initially, you both need to be submerged in the water.

For Xbox players follow this simple guide to get this achievement. Time around 5-10minutes.

Video guide -