I managed to defend the (ancient) crystal till wave 12 or so. It is fun, but repetitive after a while. Like it says in the beginning, the fire-fire-water is the most useful one. Try adding different type of enemies, with different resistance, so the player is forced to use other spells. And a spell book would be nice, to know the combinations :)
Anyway, I don't like the original Invoker that much from DotA, but this one was fun ;)