The difficulty of these levels doesn't bug me. I'll get super frustrated and keep playing without using power ups, until I figure out the trick to each level. It's a puzzle solver as much as it is a tower defense game.
My only problem is the 1000 coin cost for each re-try. Like someone said, the trial and error is half the challenge, but having to spend each retry is a pain.
I've got 150,000 coins in the bank, but I'm a miser dammit!