The Bigger They Are...

The Bigger They Are... is a Windows/Xbox 360 achievement and PS3 trophy earned by killing all 5 Super Mutant Behemoths in Fallout 3.

Locations of Behemoths

 * GNR, specifically the GNR Building Plaza (Main Quest)
 * West of Jury Street Metro by the old trains
 * Evergreen Mills
 * Capitol Building
 * In between Takoma Park and Takoma Industrial

Tips
GNR Behemoth Jury Street Metro Behemoth Evergreen Mills Behemoth Capitol Building Behemoth Takoma Industrial Behemoth
 * To find the GNR Behemoth just follow the main quest without skipping any of it and you'll come across it. (WARNING: talking to Doctor Li before you fight this behemoth can glitch your game leading to the Brotherhood of Steel soldier's corpse not being near the fountain outside of the building and thus not letting you take the Fat Man or mini-nukes from his body)
 * The Jury Street Metro Behemoth will spawn if you open the cart cage with the Teddy Bear in it [West of Jury Street Metro], but sometimes you may have to open all the cages before the Behemoth will spawn. The cart cage is inside an overturned train. Wait a bit after and you'll see it start charging at you from over the hills. It is possible that this behemoth will spawn simply by standing next to the cage.
 * In some cases, Crazy Wolfgang, Doc Hoff ect and their caravans might be killed by the Behemoth.
 * After the Behemoth spawns, cripple its legs with the dart gun and head towards the area between the radio tower and intact house back nearby Jury Street Metro Station. Doing so will result in the behemoth standing still at an invisible wall (possibly marking where the script ends its area of responsibility; XBox360 confirmed).
 * Equip plasma grenades, and start heading to where the Behemoth spawns over the rubble filled tracks going into the hills. As soon as he is within grenade range (VATS around 50% chance to hit) begin throwing as many as you can throw in VATS. This will cause the Behemoth to fly into the air, high enough that the fall will kill him.
 * Lead him to Evergreen Mills. He can make short work of the raiders and do some nice damage to the other Behemoth as they will fight.
 * The Evergreen Mills Behemoth is caged in its own electrified slaver pen so you can kill it from the cliff heads above or shooting through the fence surrounding it (The behemoth can escape if the generator by the gate of the cage is destroyed, which can be sniped from an out cropping of the cliffs south of Evergreen Mills marked by a billboard).
 * Lead him to the Jury Street Behemoth, or the Jury Street to him.
 * The easiest way to find a behemoth in the capitol building is to enter through the west entrance.
 * It is possible that the Capitol Building Behemoth will be killed by a Talon Company Merc-- you find the Behemoth and some other Super Mutants engaged in battle with Talon Mercs, one of which has a Fat Man.
 * The Behemoth will not be able to attack you in most cases if you simply stay outside the door (considering you can still hit it)
 * The Takoma Industrial Behemoth is found by heading to the left of a battle between Talon Mercs and Super Mutants to a truck with a merc on top of it and which has a switch to a Mini-Nuke artillery strike. The Takoma Industrial artillery strike can be fired numerous times but the switch may break. However, it can be repaired with a high enough repair skill level, as well as a level 33 or higher science level.

Behind the scenes
The name of this achievement/trophy is derived from the English idiom "The bigger they are, the harder they fall."

Bugs
While facing the Takoma Industrial Behemoth, you call in artillery while the Behemoth wanders. The Behemoth may teleport on-top of your head (Xbox 360)