Tips for defeating Attack at the Hospital?

I'm having trouble with the Attack at the Hospital mission:

Three lanes feel a bit too much for the amount of defenders I can field here. And I'm guessing the ice mage should go into that island in the shorter center lane, but then it suggests my defenders should be concentrated about that part of the map (since that's where the monsters are slowed), but that part of the map is not that good of a spot for the outer lanes...

And tips on how to solve this? Do I have to farm previous missions first for money and experience?