Stripped screws EMI shield PS3, can't get them out. Help please!

First of all, why do you want to assemble your motherboard?

Hereby a solution with limited damage.

The only thing left to do is to drill out the screws preferably with a small pillar drill, and use a brandnew drill the same diameter as the screw. With a pillar drill you won't slip beside the head of the screw. It is important that you do this gentle.

The only negative thing about this is you can't use the screws and holes anymore but some times you won't need them. Good luck.

If you have a very small angle grinder, such as dremel, you can use it to cut a groove suiteble for a flathead screwdriver. You would have to protect the rest of the mobo from sparks and have a very steady hand. Slip once and it's over.