Proceed through the "Post-Human" level until you leave the computer room and enter a large open area. Follow the catwalks to walkways with CELL soldiers. Fight the soldiers and kill as many as possible so they call for a helicopter to back them up. You will see its searchlight on the right side of the main causeway when it arrives. Use the Predator Bow with the strongest draw to get the best range and accuracy for your arrow. Select the explosive or electric arrow and take down the helicopter in one shot.