The power tool you would use to take the board down would be a thickness planer:
It has an adjustable cutting head and you run the board through it in successive passes to shave off the surface.
However, unless you do a lot of rough lumbar milling or are into fine woodworking, it would probably be silly to buy one. Maybe you can rent / borrow?
Note that a thickness planer is good at making 2 faces parallel, but it won't be much help if the wood is also twisted or bent along its length. The twist will just follow through the planer and you will end up with a twisted board with 2 parallel (but not planar) faces.
Edit about the suitability of thickness planer:
Several people have pointed out that to use a thickness planer you are "supposed" to have one face flat already. I guess that's optimal but I have used a thickness planer many times to "save" cupped boards. If you have a great piece of wood, you might as well try something, right?
- Start with the convex side up, so that the board is stable against the base and doesn't rock.
- Work in small passes at the beginning. If you try to take too much off the planer will press down on the board and flatten it without really cutting.
- Once you have flattened the convex side you can alternate faces.