If you've installed the Broken Steel add-on before attempting the Finding the Garden of Eden main story-line quest, there's going to be a section of the quest where you're walking through Vault 87 (I'm pretty sure it's the Lab section--it might be the Crew's Quarters, though). You're going to come across a two-story room (like in every other Vault) that'll be the second door on your left after going up a set of stairs. Once you enter, two Super Mutant Overlords will come out and blast the bejebus out of you.
This is a pre-programmed encounter that I'm guessing was with Super Mutant Masters before I decided to pay money to up my character's level cap from 20 to 30. As soon as you step into that room, the encounter activates.
So here's a hint:
Backtrack to the stairs, and go into the first room on your left. Walk across that large room, and exit through the door on the other end. After taking care of all the other Super Mutants around, enter that same room from the other side (where the Overlords would be coming from). The encounter will activate, but this time, with the original Super Mutant Masters without Tri-beam Laser Rifles. Much easier to deal with.
I hope my seventeen hours of gameplay yesterday (note: not all at that part of the game) helps you out.