The secret to getting the last 180 on a whirly is your arm and the handle. When you pop off the wake like with a regular tantrum keep the rope in tight and then send it in an uppercut motion across your chest and towards your back shoulder your head should lead the rotation. Now as you are coming around you will want to relax your arm and prepare for the landing, but wait.. That is where most people let it go and they only land it 180 or to blind. Keep your arm going across for a few more seconds and you will finish the last 180 and land just like a tantrum. Just remember to really throw your head and arm hard across and to wait a little longer than your think you should. If you happen to over rotate it just take it to a 540, stand it up and shout to the boys on the boat and tell em who the man is.
Hope this helps man