Story related, cannot be missed. Weapon 11 is the final boss of the game, you will encounter him at the end of level 5. Tips on defeating Deadpool: he is the last boss and will give you some serious trouble, especially on hard... unless you fight cheap.
The surefire way to kill him without issue is to charge a heavy attack (hold ) and use it to launch him in the air. Jump and stab him 5-6 times but DO NOT finish the combo. If you do, it will send him flying off the side, but he will teleport back and be far away from you. Instead, let him fall to the ground. Immediately charge another attack with and repeat this process until he is dead. If he does get away, move toward him quickly and lunge to him. Start back with the cheap aerial assault as soon as you can.