Riddle #3: "Be lion-mettled, proud; and take no care Who chafes, who frets, or where conspirers are: Macbeth shall never vanquish'd be until Great Birnam wood to high Dunsinane hill Shall come against him." (IV.i.90-94)

The riddle states that Macbeth shall never be overthrown until the Great Birnam wood moves to Dunsinane hill and closes in on Macbeth's ground creating pressure and closure around the castle. Since it is impossible to move a forest form one place to another, I believe the only way for this to be possible is if Macduff's army disguises themselves as the forest with their weapons in hand and trick Macbeth into believing that the forest has actually moved, so that he has a sense of being trapped. When the army is close enough they can surprise attack and succeed at capturing Macbeth. I think this is the only possible answer as I don't see how Macduff's army would be able to move a forest from one place to another such a long ways and not be seen (blow their cover) by Macbeth.