[Bergil]

No-one had an easy job of Killing them because no-one expected an air force. someone suggested an anti-air ballista, that might work. The suggestion of a bolas also holds water. But the best idea would be a trap. Lure 1 away from the others with an easy target such as a lone rider, seemingly a scout far from any help (the Nazgul seem very organized to me), then knock him out of the sky with the catapult you're hiding. This will also temporarily squish the Ringwraith.

Edit: the Fell beast will also be lured close to the ground