When fighting most of the bosses in LEGO Lord of the Rings, you should concentrate on dodging their attacks. All the bosses will give you a window to allow you to attack and, as long as you are patient, you will be able to attack with less risk to yourself. You can also hold the character selection wheel open while you are waiting for the window, as you are invincible with it open!
If you are playing the Mount Doom Gollum fight, it is possible for it to glitch if you are using the Invincibility Red Brick. Bear this in mind if fighting Gollum that you may be better off avoiding using this brick.