This trick might be a little hard. Play any level. When you find the
exit, go to it. It will tell you how many treasures you have found
and secret areas. If you didn't get all of the treasures and
secrets, press B to go back in the the mission. Look for all the
secrets and treasure. When you think you found them all, go back to
the exit. If you found all of them, on the bottom screen will say
"You found all the secrets. You receive..." And it will tell you
which weapon you have won.