Thou losest labor.
As easy mayst thou the intrenchant air
With thy keen sword impress as make me bleed:
Let fall thy blade on vulnerable crests;
I bear a charmed life, which must not yield
To one of woman born.
Macbeth tells Macduff not to waste his time. It would be as easy, Macbeth says, to make a cut ("impress... with thy keen sword... the intrenchant air") on the air, as to make Macbeth bleed. Macbeth has a "charmed life", he says and no-one born of a woman can kill him.
Despair thy charm,
And let the angel whom thou still hast served
Tell thee, Macduff was from his mother's womb
Macduff tells Macbeth to forget about his "charmed life". Macduff was, he explains, cut out of his mother's womb before he could be naturally born ("untimely ripp'd... from his mother's womb").
Accursed be that tongue that tells me so,
For it hath cow'd my better part of man!
Macbeth is defeated - and cowed - by this information. He despairs of the witches' advice and prophecies. Eventually, he resolves to fight ("Lay on Macduff!") anyway - and is killed.