You simply need to get to the door behind El Gigante, and not neccessarily kill him, although obviously that makes it a lot easier to do so.
As for the chainsaw sisters, see if you can get a rocket launcher from the merchant, then try and line up a shot that'll take them both out. Again, you can get past that section without killing the bosses; they just drop a buttload of cash if you do. In fact, I usually did one path, then went back and did the other one, to get as much cash as possible.