I did this on Psychotic difficulty. Take out all 6 bosses and then go for the Final boss on the oil rig. Now this oil rig boss you make think is tough, but there is a way you can beat him in less than two minutes.
There are three platform in the water, all connected through pipes.
1) Equip a sniper rifle and an assault rifle
2) When on the main island, jump into the water and swim out to the 3 rig(the main oil rig with the boss on it). There should be and opening in the back of it just above the water.
3) There is probably about 5 guys inside, kill all of them. Now jump from platform to platform inside the oil rig, till you get about half way up.
4) You should see some pipes that go out from the inside of the oil rig, to platform 2. Jump on the pipes and run out to platform two.
5) Now get to the very top of platform two, kill everyone you can along the way. (Easiest, just kick them over the sides into the water).
6)Once you reach the top wait a few seconds to make sure no one followed you up, if they do, kill them.
7) Now this is the easy part. The boss is at the very top of platform 3(the oil rig). Use left trigger to look for him, he will be at the right corner of the platform that has the crane coming out of it. This is higher than the helicopter pad. You should be able to lock onto him.
8) Take out your sniper rifle and cap him. On psychotic difficulty it took 15 bullets. I tried doing this with a rocket launcher, but every time you fire, he runs away. If you use the sniper rifle, he just stands there and lets you shoot him and never moves.
There is a shortcut to almost every boss. If you need a description on how to take out another boss quickly just send me a PM!