So 7 out of 8 of the black screws on the back of the metal case holding the motherboard are stripped. I mean completely stripped, just a small circle. My guess is that they got stripped from the xbox repair technician when I sent it in about 5 years ago when it was under warranty. Needless to say it has the RROD again.

Does anyone know what's the best or cheapest way to remove those screws?

I've got thermal paste ready, new x-clamps screws that I brought online ready, I just need access to the X-clamps.

Wrong forum mate, you want the 360 forum - oh and you will need to grind the heads off and then use multigrips to remove the remaing screws.