This can only be unlocked with Sam B and after unlocking his Tackle ability. Play Chapter 16, "Locked Up". Once you reach the checkpoint where you have to "Find another way to the cell block control room", you must go around to get to the control room. When you open the door to the cell blocks, there is a stairway going down and there you will see the Ram. Allow him to follow you upstairs and go back in the hallway. Leave the door open. The Ram will try to ram you down, but he will not be able to get past the door. This is where you can weaken him until his life is close to dead. Then, deliver the killing blows with you Tackle ability.