Film is a visual medium. Don't you think the killing blow had to be visually spectacular to illustrate to the audience what was going on? An invisible power struggle worked once (the wizard's duel at Orthanc), but for the balrog, we needed to actually see Gandalf gather power and then strike. Besides, if they are made of the same stuff, and one has a physical form that can be damaged (Gandalf), then the other one has to too
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