How to win when defeating Bulls Eye? When you are in King Pin's office, try to throw grenades at him instead of shooting. You have to keep throwing grenades until he will be defeated.
How to defeat the chopper at Stark Towers? Actually, when you play this part of the level and if you want to get out of it as quickly as it possible, run to the middle rock near the edge. Find there an anti-tank launcher and try to aim for the man with the rocket launcher in the chopper. After you defeat him, keep firing at the chopper.
How to defeat Jigsaw? You have to take out Jigsaw's jet pack, first and then walk over to a cargo crate. Jigsaw will come to you if you are patient enough and wait for a little while. Run under him when he is near you. Shoot his backpack and stay under Jigsaw. Then you have to run in between boxes to throw him off your path. Run around until you see a guard, then finish him quick. The guard will drop a RAM explosive. Jigsaw will run after you and when he gets near the RAM, press your Secondary Fire. Repeat this procedure until Jigsaw is defeated.