Ironically, i just got the trophey for this like 10 minutes ago.
The first one is right on the main storyline. When you get to GNR.
The second one is at Evergreen Mills, which is a bit northeast from Smith Casey's Garage (Tranquility Lane). It's in a cage by TONS of raiders.
The Third one is northwest of Evergreen Mills. There are a bunch of crashed train cars there and a few mutants. Kill them, and open the shopping cart cages, and the behemoth will come.
The Fourth one is at Takoma park, witch you will have to look up, because It's hard to explain. There is many Merc's/Regulators there, and a few super mutants. There is a rampt that leads you on top of a truck, that has 3 first aid boxes and a switch that fires artillery beside the behemoth. You can use it, or your own guns to kill it.
The fifth one is inside the capital building's dome. you can just wait until the Talon Company Merc's get his health down most of the way, because they have a fat man.
Enjoy :D -Tmac