That is a horid job, cutting of a quarter panel and then reinstalling it, yeow, that hurts. Cutting off small panels, braces or portions of panels is common though. So it isn't out of the question, just uncommon that you would need to remove a quarter. Are you sure you couldn't remove a portion of it, like the up to below a body line or something like that? Post some pictures!
Sometimes removing the whole thing would be easier, other times just a portion may be a big time saver AND less "invasive" and better for the project all the way around.
But getting back to the actual removal, cutting with a cut off wheel is one tool that could be used. You would want a 1/32" wheel, that is about half a mm. But the method most commonly used would be drilling out the spot welds and removing it just the opposite of how it was installed.
Some people will get wierd about drilling out spot welds, it is really no big deal. Look at it as "unbolting" the panel. Those spot welds are really no more than a "rivit" or "bolt" if you think about it, just carefully drill the top surface off, and wham, you have "unbolted" that point.
But you need the proper tools for the job. Over at Autobodystore.com and get some "Wivco" spot weld drilling bits. These bits have a very short pilot drill and cut off the top metal layer only. They are very nice, but I have to warn you, you have to go slow, have the drill perfectly right angle with the surface and start a little pilot "dent" with an 1/8" drill is recommended so the Wivco doesn't wander. If it wanders, it is likely to break. They are VERY hard steel and will fracture pretty easy. But if you are careful one will stay sharp.
This is what a Wivco bit will leave of a spot weld.... (though I didn't use a Wivco to do it, I used another brand)
This is the bit, it has a retractable tip. And you will see that it has very shallow, flat cutting surface so you don't go thru the bottom layer of metal.
As I said, this is like "unbolting" the panel. If you go around and drill the spot welds out like this, the panel comes right almost as if it is "unbolted". There are a few welds that will have a spot that didn't get cut, and you use a panel splitter or my favorite tool for this a gasket scraper to carefully split the panels apart.
This is a "gasket scraper" like I use to split the panel, AFTER the spot weld has been drilled out.
I will often preach about using tools correctly, but this is one time I break my own rule. These gasket scrapers are GREAT tools for splitting panels. They are VERY thin at the end and don't distort the metal around the spot weld. I have found that the Snap On brand tool is the best hands down, I have been using one for YEARS beating it with a hammer to shave pieces of stubborn spot welds and it is still going strong. I bought a "MAC" (another big Tool truck brand like Snap On) to bring home so I could leave the Snap On at work and found that the handle broke from the abuse. The Snap On one is still going strong. But I have tried the tools sold as "Panel spliters" and don't like them at all, they are too thick.