The Golden Compass

Story related. Once you bring back Iorek his helmut he'll go out for vengence on the tank. Just run around the tank, wait for it to have an electrical surge, once it is not glowing with electricity you can go to it, push it over(follow the button prompt) then slash away. Rinse and repeat like 4-5 times depending on how many slashes you get in each time. Key is to stay away and move around.