More information regarding Super Mutant Behemoths:
The only way to reach Takoma Industrial is through the metro. The only entrance to the metro that is linked to Takoma is Abernathy.
I found the number of Mini-Nukes it takes to kill a Behemoth per difficulty setting. The research was done at GNR using the original Fat Man. A Fat Man with better condition may take one less nuke on the easy difficulty setting, but the rest have been confirmed at a later point with a very pristine Fat Man.
The number of nukes (direct hits to the chest) is as follows:
On Very Easy: 1 Nuke
On Easy: 2 Nukes (1 if Fat Man is well maintained)
On Normal: 3 Nukes
On Hard: 3 Nukes
On Very Hard: 4 Nukes
An expierience guide:
On Very Easy: 25 XP (1x)
On Easy: 37 XP (1.5x)
On Normal: 50 XP (2x)
On Hard: 62 XP (2.5x)
On Very Hard: 75 XP (3x)
Some advice on damage taken (from your own nukes):
If you are cornered, use V.A.T.S. and target the head. This does minimal damage to yourself, and more to the Behemoth. When firing at the head, I only took about 20-60 damage out of my 290 heath max (on Normal Difficulty with full heath when firing and 18 armor). When firing at the legs/ground at a very similar distance, I was killed every time. Abdomen shots either killed me or left me with minimal health (0-50). Even upper torso/arms left me with 70-120. On any difficulty I advise only headshots if you are within 50 feet of the behemoth. As the difficulty increases, so does the advised distance. Even on Very Hard, headshots aren't fatal if you have more than about 80 health. A ground/leg shot on Very Hard was deadly within about 140 feet. When I had the Power Armor though, with full health (390), I could withstand a direct Nuke on Hard, so I think the information above is more helpful to anyone having problems with backlash from their own Nukes.
I hope that my experiments, data and other information can help you fight the Good Fight, and become a successful Behemoth head hunter. If you have any ideas for further testing, want to know my testing conditions, or anything else, just PM me, and I'll try to help.