The Pain is one of the 1st bosses you will fight. There are two ways of beating him; lethally or non lethally. I recommend that you defeat him non lethally (stamina depletion) which allows you to gain his uniform "Hornet Stripe" which is one of the camouflages you need for Fashionista. The Pain isn't very hard to beat. He usually throws grenades at you which attract the hornets or shoots you with a machine gun. He is protected from hornets but you can shake them off by throwing smoke grenades at him. Then shoot him with the tranquilizer until you beat him.