I also tried this on the hard setting more than 100 times without success. Here's 2 screens of how I finally managed to win it (Flash/PC/1.13):
http://i68.tinypic.com/2na89s1.jpg
under the PAUSE notice is a barracks and a ranger tower
http://i66.tinypic.com/2u4ijow.jpg
Couple of notes. If you made it this far in the game, you probably know, but just in case, just a couple points:
(1) I had to buy and sell some towers, as you can see. Some levels you can just buy towers and sit on them, but I had to swap to get through this one.
(2) There was a horde of skeletons and imps that made it right to the doorstep. I was watching it and expected another loss. It was really that close. If you see your soldiers from the barracks fall, upgrade and new ones will come right out. Same thing with the wizard's elementals, which seem to be pretty good at defending that area. Watch the elemental's health, and when it gets low, before it crumbles, bump up that tower's Special Abilities/Elemental setting one level. When you do that, the elemental's health instantly gets 100% refreshed.
(3) For the wave of magma elementals at the end, I bought a musketeer tower with the sniper upgrade.
(4) It is important to get those 4 necromancers early. If you put a barracks in their way, at that top-right plot, they'll stop when they come out and start their ranged attack. That's fine, wait until they're all out, then drop the meteors on them, when the 4 are grouped. If one or more of them survive, keep after them. Keep placing reinforcements to attack them. Without artillery, they're a lot of trouble.
(5) Instead of placing reinforcements in the path of the skeletons emerging out of the graveyard, place your reinforcements just to the right. They will stay alive much longer, and have their ranged spear attack to hit the skeletons with.
http://i68.tinypic.com/n4ebz8.jpg
(edit: I apparently got the forum code for images wrong)