Ok this is the toughest level in the game.  Use Legolas for his good bow
skills and speed.
  First kill 60 guys.  Then you will get a checkpoint.  You'll have to
kill an elephant while your up there.  Go to the side where it has a
catapult.  Shoot it with the catapult and then shoot it with arrows.  It
will explode then you will have to kill another elephant.  Go to the
other side and do the same thing.  Then it shows Merry and Aowen being
knocked down by the witch king.  Run to the other side of the mountain
and load up your arrows.  Let the arrow go after a while but not too
long or he'll fly away.  Then you'll have to kill another elephant and
then the witch king again.  Make sure the witch king doesn't kill Merry
and Aowen or you will fail the mission at the checkpoint again.  It will
keep on doing this until you die, Merry and Aowen die, or the witch king
dies