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The Super Mutants will be non-hostile to start with but as soon as you enter the Watershed building you will be accosted by Harry. Take advantage of Harry's stupidity and persuade him (with some speech skill) that you are a robot, otherwise you will find yourself carted off to the Military Base, which is an even more difficult challenge. If successful, you can now move around the Watershed freely. Check the shelves behind Harry for weapons and ammo, including an Assault rifle which, if you don't have anything more powerful at this point, will be essential to you now. Concentrate your fire on Harry. He is tough and has a lot of damage resistance as well as a very dangerous laser rifle. Use burst fire mode with the assault rifle to take down Harry as quickly as possible. After him, the rest are easy. Bottleneck them in the doorways and take them out, though watch out for the 'female' Super Mutant's flamethrower (in Fallout 2, she is described as having killed Ian). However, if you do have some useful weapons, like the Combat shotgun, you can take out Harry with ease, by shooting with burst, then occasionally checking for his HP if you have awareness. If he seems to have low hp, go near him, and be sure to have enough AP to shoot him down with burst, then do the same as above.