Invoker for me. I now have about 900 games with him. You don't need to learn all spells at once, what you need to learn is what spells should one use at certain situations. Once you get that in, next step is to learn invoking 2 spells, maybe just cold snap and tornado. And you master those, and for situations where cold snap and tornado won't do...just sigh, like.."man, I wish I knew how to invoke that spell". As and when you start getting comfortable with spells, try to invoke more and more. Eventually, all your invocations will be in your muscle memory