Instructions
Finish the beginning of the story. The Fallout 3 storyline will not let the user progress eastward toward Takoma Industrial without being sufficiently advanced. You have to completed the tutorial, have taken the G.O.A.T. exam and escaped the Vault. Going to Megaton and GNR radio is recommended beforehand. This will prepare one's player for the challenges presented by both a hostile wasteland and a super mutant behemoth.
Head to Vernon Square. Vernon Square is a section of postapocalyptic Washington D.C. located right next to the Vault-Tec headquarters. You can get there by crossing the radiated river, heading to Galaxy News Radio, which you will have to go to during the main storyline, and going north to Chevy Chase East metro station. Inside of the metro there will be a door indicating "Vernon Square. " Enter it.
Head through the Vernon East/Takoma Park tunnel. Fight your way through the Vernon Square super mutants and make your way northeast from the metro entrance. The Vault-Tec headquarters is right next to Vernon East/Takoma Park. When inside you'll notice the passageway is partly caved in, albeit still accessible. It leads down to the singular Takoma Park exit. Your goal is to aim for the northeast corner of the map, following the rubble along the right hand side so as not to get led astray.
Go to Takoma Industrial. A subsection of the larger Takoma Park, Takoma Industrial's entrance is located to the north of the metro station's exit. Be aware of the bountiful super mutant grunts and overlords present in the area while traversing it. Pass up Nifty Thrifty and Takoma Motor on your way northward. Enter the "Takoma Industrial" labeled house on the left hand side of the street.
Find and battle the behemoth. The super mutant behemoth will be a challenge to take down, so be prepared with an ample amount of stimpaks and meds. Walk to the southwest side of the compound, take out the behemoth's super mutant friends and open the gate. Get on top of the truck next to the dead mercenary and press the artillery strike button. This should shower the beast with some critical damage.