I have a round griddle that was warped enough it was almost un-usable on my glass top stove. Here's what I did.

Using a ruler and an angle grinder, I knocked down the high spots. When I put it on the stove again it was significantly better but still a bit wobbly so I took a cheap 2-sided sharpening stone and continued taking it down with that. It took a few hours to get as flat as I wanted but, I felt like it was worth it.