i kileld one caged up at evergreen mills and a palidan got teh last shot on the one i was killing during a quest outside of GNR station (hopefully that wont count against the achievment)

i keep hearign one around a shopping cart where is this shopping cart ???

i talked to a kid at the citadel he said he saw one near the citadel is there honestly a behomeneth out around teh citadel or did teh kid just see a normal mutie ?

All http://fallout.wikia.com/wiki/Super_Mutant_Behemoth locations.

great ive passed by the trains with shopping cart cages over 60 times :/

i went into the capital building and left :/ or maybe i cleared the place i forget

and only place that will be new to me is takohoma ^.^

slaying i shall go ???

You have to do a bit more exploring to find the White House behemoth. It's downstairs in a huge open room. You'll want to be careful when you go down there - there's a huge fight between the Talon Mercs and the supermutants going on down there and you'll have no allies beyond those you bring with you - not like the GNR fight when you had the BoS helping out.

fixed to avoid confusion^^

That one's inside the Rotunda of the Capitol Building, and he's actually pretty easy to defeat. He's too big to fit through the doors leading out of the Rotunda, so if you stay in the hallways, he can't reach you and you can just chip away at him until he crumbles.

will i still be able to see its head ??? i always go for the head


(on a side note how can soem raiders with out helmets survive 6-9 shots to the brain and still be walking??)

Yes, and as mentioned above, its pretty easy to bring down that one.