You HAVE to have A/C for an attic grow in the summer. A portable A/C unit, fans and a grow room insulated to beat hell will make it possible but still difficult to be successful. Also, besides fans inside your grow room, you need a strong fan in a gable vent to pull heat out of the attic. Stick-build the grow room, and then insulate it being sure to make it as airtight as possible. You basically have to build a room within a room to be successful. Make sure to run your lights at night ... say on at 6pm off by noon for an 18/6 veg cycle or on at 6pm off at 6am for flower cycle. Or, maybe try growing 12/12 from seed ... less lighting, less heat. That being said, if you are having continuous 90-100 degree days and nights that don't cool off ... you probably will never get ahead of it because it will be 120+ in the attic.
Lastly, (and this depends on where you live and how close to other homes) if you are able to eliminate using a carbon filter for your exhaust and vent directly to a roof vent, you can move a greater amount of air through the room. This will help tremendously in you efforts to cool the room.
You are gonna run into the reverse in winter if it gets cold where you live, so start thinking about that scenerio now. The more airtight and insulated the grow room, the better chance of success. I'm not pulling these suggestions out of my ass ... I have experience in this.