Macbeth and Macduff are actually fighting when Macbeth says:
I bear a charmed life, which must not yield
To one of woman born.
Macbeth has come to believe implicitly in that the three witches have told him, because their previous predictions have come true. But Macduff's response comes as a surprise and a shock.
Despair thy charm,
And let the angel whom thou still hast served
Tell thee Macduff was from his mother's womb
Macbeth refuses to continue fighting. He still believes in the witches' predictions and feels certain that Macduff is predestined to kill him. But Macduff is determined to fight to the death. He tells his enemy that he must either surrender or defend himself and that if he surrenders he will be kept in a cage and exhibited to the populace with a sign reading "Here may you see the tyrant."
Macbeth, of course, cannot endure that, so he continues with the death duel. No doubt Macduff succeeds in killing him because he has the stronger motivation and because Macbeth has become unnerved by beginning to understand that the witches' prophecies have double meanings.
And be these juggling fiends no more believed
That palter with us in a double sense,
That keep the word of promise to our ear
And break it to our hope.