So when you go in the multiplayer tab for Hexen, go to local, and you pretty much don't need to change any of the settings that are given to you. Just start up the co-op match.

Once you're in, press the ` (or ~) key to open the console. Type in "addlocalplayer" twice, it'll add in two "players" with your name and a number at the end.

Still on the console, type "exitlevel" six times until you're at the ending cutscene and the achievement pops up.

It's as shrimple as that!