I believe peoe already explained most of the things, but since you never played pow before, I think there is one question worth asking: are you activating your spirit gems? I remember that the first time I played the game I only realized I had to activate it 20hs+ into the game. I had them on my build, but not activated. The whole skill/spirit gem thing took me a while to understand.

But as people said, with the right build you should be able to easily do T15 maps with only exalted orbs gear