The Bigger They Are...

The Bigger They Are... is a PC/Xbox 360 achievement and PS3 trophy earned by killing all 5 Super Mutant Behemoths in the game.

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 Indusrial Behemoth
 * To find the GNR Behemoth just follow the main quest without skipping any of it and you'll come across it.
 * 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 and his caravan 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.
 * 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).
 * 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)

Tips for all Behemoths
 * 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.
 * Use the Fat Man
 * A Fat Man may take up to 3 shots to kill a behemoth, while the Experimental MIRV will kill anything in one shot. Of course, the MIRV is not very practical to kill anything with considering it fires 8 mini nukes at a time, which is a complete waste of ammo when you could just use a normal fatman to shoot it 2-3 times, depending on your big guns skill. But it would be effective.
 * If Fawkes can be moved to a safe location, he can quite quickly kill a Behemoth.

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

Bugs

 * There are times when a super mutant behemoth that is not connected to the "the bigger they are..." achievement might spawn(if this isn't a bug, please move to notes)

Evergreen Mills

 * Sometimes the Behemoth may disappear and appear only when you will destroy all nearby generators.
 * Sometimes, upon freeing slaves, they may free the Behemoth as well. Unsure if it is scripted this way.
 * Sometimes, a fleeing slaver will open the pen and hide inside, freeing the Behemoth.