Who needs UO? lol!

I'm addicted to Defender.
I keep playing it of a lunchtime at work.
I can complete it on easy and medium, but I'm struggling on Hard...
Any tips?
Got to be really keen on setting your chars up right. They get special abilities when you train specialized, they get raw power when you train general. For instance, fire specialized gets splash, that means when they cast they hit everything in a radius doing damage to many creatures at once. A good strategy is to put a freeze specialized mage that slows monsters down just before they get to a big pile of genralized fire mages that have the highest raw damage output. Also, its best to put things where they have reach of monsters for the longest period of time (sometimes monsters walk around corners a few times circling around a char, better to put your chars there than in a spot where they only reach for 3 tiles).