Don't have any nukes at hand, as much as you'd like to use them?
Now, just use this Flatificator program for the Terraformer. It will quickly eradicate all naturally formed blocks ABOVE the height level of the placed terraformer.
It cannot remove man-made materials, though.
As well, it will fill all areas below the Terraformer's height level with simple dirt. Means, it can flatificate BOTH WAYS, HAYO!
See: Crafting Guide
Input: Needs a minimum of 40 EU/s
Further InfoFlattens an area of about 60 by 60 meters.