Destroy the Super Mutants at the Watershed

You need to start this quest before 40 game days have passed or all of the Ghouls in Necropolis will have disappeared.

If you talk to Set, the leader of the Necropolis ghouls and manage not to anger him, he will give you the opprtunity to work for him. He wants you to kill the Super Mutants in the Watershed area. Go there through the sewers and prepare for a difficult fight.

Tactics
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.

There are a total of 6 Super Mutants in the area. Once all are dead, return to Set in the evening to receive your frankly paltry reward. If you then ask Set for the information he also promised tread lightly as he could turn on you for it and you'd have to fight all of his ghouls.

Reward
40 bottle caps.