Start the Say Hello To My Little Friend level, and go around to the right instead of to the left. You will pass the house (from outside the fence) in which Pablo is in (second floor, in the room closet to you). After you are just slightly to right of the house (on the outside of the fence), you can snipe Pablo in the head and kill him with one shot, which is much easier then fighting him normally.
Start the Say Hello To My Little Friend level, and go to where he is located. Get behind him and make sure you have the knife ready. You can slit his throat and it will end the battle.