After the Fellowship is ambushed by Orcs and you defeat some of them whomever you're playing as will be pursued by the Troll, after giving him several blows your character will jump onto a ledge and be followed by the Troll. Shoot arrows at him
and hide behind pillars when he swings his chain at you. Some orcs will be on the ledge as well, defeat them and get posions which increase your health, just keep shooting that Cave Troll and duck behind pillars and you'll get him!