SUPER MUTANT BEHEMOTHS!

There are only 5 super mutant behemoths in the Fallout 3 game. They are found in The Capitol Building, Takoma Industries, Evergreen Mills, GNR Building (Galaxy News Radio), and the Jury Street Metro...
They are the toughest enemies to take down in the game. In every location that you find them in, you'll have some help. Brotherhood of Steel, Talon Company, and turrets that you can activate will make it a lot easier to take down all five. If you do take down all five of them, you will get an achievement called the bigger they are...

Good luck...