how do you win bloons tower defense 4 on hard?

Answer 1

There are many different routes you can take to beat the game.  However in order to succesfully beat it you should have an acount on mochigames. Explaining the stragety is a little difficult so I'm sure that this video explaining everything should suffice.

http://www.youtube.com/watch?v=uiU8xWuBmq0

Answer 2

I have put waay too much time into this and am pretty confident in my answer. The trick is to damage the bloons early w/ aoe then pick them apart with your cheap towers closer to the end. Use the spike launchers to whittle them down and then upgrade a few dart monkeys near the end.