http://fallout.wikia.com/wiki/Super_Mutant_Behemoth for Behemoth:
* The Behemoth right outside the entrance to the GNR Building is the easiest to find and is actually impossible to miss if you attempt Galaxy News Radio (quest). You will have ten Brotherhood of Steel members to back you up, though they are not invincible and most will likely be killed quickly. One of the Brotherhood members is in possession of a Fat Man weapon and will die quickly, before having a chance to use it. The lamppost in the area is fragile-looking, but the Behemoth considers it a wall, and it can serve as effective cover from the Behemoth. If you have trouble defeating this behemoth it’s possible just to run to door of the GNR building. There is a back exit to this building so you will not have to face him on the way out. Also when you quick travel back to this location you will be placed behind the sandbags pretty much out of harms way near the door. Another possible strategy is to take cover under the GNR building, coming out when your AP recharges to use VATS.
Behemoth in Evergreen Mills meets a Fat Man from the rooftops
Behemoth in Evergreen Mills meets a Fat Man from the rooftops
* In Evergreen Mills, the Raider-controlled train yard, a behemoth is trapped by an electric fence. When approaching the area from the south, the generator can be easily destroyed with any rifle; a melee will ensue in which most of the raiders will die killing the behemoth. However, no experience will be gained by this method. The easiest method of legitimately defeating the behemoth is to clear the area of raiders, lay several Bottle Cap Mines near the gate(but far enough away from the generator so they don't detonate when it explodes) and then release the locks on the cage. Alternatively, the Behemoth can be attacked by ranged weapons through the mesh gate while the generator is still active. The roof of the nearby train car offers a clear view of him. However, it is possible that the Behemoth may escape during the player's shootout with the Raiders, forcing the player to deal with him in the tight confines of the camp. Another alternative is to jump between the carriages to get to the other side of the carriages. The behemoth will try to get through but can't and eventually will run off. Just jump back over and it comes back, repeat until the Behemoth dies. Normally the Behemoth won't attack any followers with this method.
* A Behemoth can be found in the Capitol Building fighting Talon Company mercenaries at the west entrance. There is a considerable amount of fighting between Super Mutants and Talon Company so its best to just sit back and watch (or snipe if you want the experince points). After a bit you'll find the room under the dome and that is where the Behemoth is waiting. He should be getting shot at by more Talon Company so just wait until he has low health and hit him with one nuke.
* A Behemoth is located southwest of the Takoma Industrial in Takoma Park. There is a truck near a water tower a few feet away. On the truck you can see a dead mercenary and a switch that activates some artillery that makes the fight a lot easier.
* One final Behemoth is located at a Super Mutant camp, after following a series of search party notes (Unmarked quest: Searching for Cheryl). The final note takes you to a camp between the Jury Street Metro Station and Evergreen Mills. Both are located due west of Vault 101. You do not need to get these notes to make this Behemoth appear. Keep going west from Vault 101 until you find Jury Street Metro Station. Head further west from there until you find a bunch of wrecked train cars along with a few Super Mutants. If you're having trouble finding it follow the tracks heading NE from Evergreen Mills. The Behemoth should appear after you enter a carriage filled with shopping cart. Locate two carts with a teddy bear inside and open the "cart cage". Finally taking you to the final location where the last (and hardest to find) Behemoth should have spawned.
Note: You can wait up on the edge overlooking the train carts and wait 12 hours and the behemoth will appear, but sometimes Enclave soldiers may appear as well. They will ally with you against the behemoth but once he dies they turn on you. Fawkes can be helpful here as well.
It's also possible to snipe with Behemoth from the cliffs near the entrance to Evergreen Mills. He won't be able to trace the fire back to you and will continue to schuffle around. Blackhawk is a good weapon for this job.
If you want to resurrect the Behemoth from GNR building for another fight, use ID# 1F1F9 (make sure the roof is high).