i got a simple algorithm for my wave system. But i do not have multiple monsters at this point.
<img src="http://i.imgur.com/XhzcucSl.jpg" border="0" />
What you can do is always spawn round(currentwave/3) and spawn that amount of each monster or something. You can get far with a bit of math. And maybe start that when currentwave > 10 or something.
i also do monster -> set HP = currentwave*5