Greased Lightning is a unique Power Fist that can be purchased from Knight Torres, a Brotherhood of Steel quartermaster located inside the Brotherhood of Steel bunker.

Quick tips before you go get the weapon:

• Bring Veronica. If this is your first time visiting the Brotherhood of Steel, bringing Veronica will allow you to skip an annoying sidequest that can lead to you losing reputation with the NCR. If you don’t know how to recruit her, check out the guide here.
• Progress in the Brotherhood of Steel questline required. Knight Torres will not trade with you until a certain point in the Brotherhood of Steel questline. For info on that quest, check out our guide on the “Still in the Dark” quest here.
• Expensive. This weapon costs 15,000 caps or more.
  • Can be circumvented with the ghost dupe glitch. Ghost duping is a glitch that allows you to avoid paying the high price for the SMG. Check out the guide here for more info.
• Gun Runners’ Arsenal DLC Required. To get this weapon, you’ll need to have the Gun Runners’ Arsenal DLC installed.

Acquisition

Start off by going to Hidden Valley, located directly east of Goodsprings.

Once there, go to the westernmost bunker – that’s the bunker where the Brotherhood of Steel resides.

Enter the bunker and go down the stairs. You’ll see a door in front of you. Go inside, or let Veronica open it for you if this is your first time here.

Go through the next door and down the next set of stairs that lie beyond it.

Once down the stairs, go through the door on your right.

Turn right again and follow the corridor.

There’s a shooting range at the end of the corridor. Go through and turn right again.

Knight Torres will be inside the kiosk. 

She’ll sell you the weapon for 15,000 caps or so.

Comparing Greased Lightning to the Power Fist

• Deals less damage than the Power Fist (32 damage vs 40 damage, or 48 damage if you have the fully upgraded Power Fist).
• Deals less critical hit damage than the Power Fist (32 critical hit damage vs 40 critical hit damage).
• Attacks much faster (3 attacks/second vs 1.09 attacks/second, or 1.31 attacks/second if you have the fully upgraded Power Fist).
• It’s far more durable (995 attacks before breaking vs 395 attacks before breaking, or 795 attacks before breaking if you have the fully upgraded Power Fist).

Gameplay Tips

• While the weapon is powerful, it’s basically a worse version of the Industrial Hand from the Lonesome Road DLC, as it deals less damage, attacks slower, doesn’t ignore DT or DR like the Industrial Hand does, is less durable, and must be purchased for an insane number of caps.
• If you don’t want to get the weapon from Torres, you can kill Torres, and the Gun Runner’s Vendortron may have it on sale, though a bug prevents this from always happening correctly.