Instructions
Parry the attacks of Gollum when he attacks you. Frodo cannot damage Gollum so attacking at this point is pointless.
Push Gollum towards the edge. Even though you can't attack, you can parry your way to the edge. Once there, hit him to make him lose his balance.
Use a fierce attack on Gollum while he is off-balance. This will push him off the ledge but not completely, since he will still manage to hang on. Use your killing move for him to fall.
Repeat the steps above as Gollum will climb back up and try to kill you once again. After you lead Gollum to the edge, Sam will hit him and he will be off-balance once more. Use your fierce attack and follow it up with your killing move.
Dodge the falling rocks that try to squash you. You must repeat the entire process five times before Gollum is finally defeated.