Don't have nukes at hands, as well?
Now, just use this Flatificator program for the Terraformer. It will fastly eradicate all natural 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 Terraformers height level with simple dirt. Means, it can flatificate BOTH WAYS, HAYO!
See: Crafting Guide
Input: Needs a minimum of 40 EU/s
Flattens an area that may be bigger than 20 by 20 meters, and only works above ground.