How to remove very stuck stripped screws?
Locking pliers, which is the generic term for a class of product often called by the name of one registered trademark (Vise Grips® - no particular endorsement implied, but some of the cheaper knock-offs are quite useless. I believe Mole Grips® in the UK)
You need them clamped on hard enough that they actually bite into the screw head. With that presentation, you'll only get a fraction of a turn at a time. And depending on how hard they are stuck, you might break the screw (at which point you drill around the screw with a hollow bit and remove the wood plug and screw carcass together)