Going Under

After fighting your way through the first 3 main dungeons (Joblin, Styxcoin, Winkydink) and defeating their bosses, a portal will open up in the center of the main office leading to an area underworld. You will now have a mission on your itinerary list saying "DO NOT ENTER THE PORTAL".

This is actually the main storyline and you should go into the portal against that tool, Marv's wishes. You will fight through 14 basement rooms with some of these being shops, culminating in a boss fight against Marv. He has a sword and does a bunch of teleport and slash moves. If you have Endorsed the "Fiery Glare" skill, you can simply keep focusing on him from a safe distance with LT and watch his health go down while dodge rolling away from his attacks. If you manage to pick up the skill where dodge rolling leaves a trail of fire or drops bombs, you'll have an even easier time. Try to slash at him with the sword when you have the opportunity and then dodge away. This is also much easier if you turn all the assists/cheats on from the main menu.

After defeating Marv, there will be a cutscene and some dialogue. The achievement should pop here.