Razor blade scraper.

Did you try a cooktop cleaner like Weiman's (or other brand)? I used the Weiman's with their "scrubby" pad, and the cooktop looked like new.

I've heard that those Mr. Clean magic erasers are good for cleaning those glass top stoves, but probably more for maintenance than the crusted on stuff you are describing. Good luck, I hope you can get something to work!

If it's been chronically neglected it's likely permanent. As mentioned above, razor blade and cooktop cleaner. Brand makes a difference. In my experience Whink is the the best but I can't find it anymore. My store replaced it with Weiman's which I like least of the three I've tried. Ceramabryte ranks in the middle.