[soah]

I've beaten Hard several times in a row. Load up the left and right with Darts and Cannons where they can hit both paths, and a bunch of Darts in the lower middle (I upgrade range on these, but not the others... and do the other upgrade on all of them). I also put one Cannon and a couple Darts in the upper right area where they can hit the bloons at the very start of the path... in order to get started on the black and lead bloons. A few levels require a bit of Monkey Glue.