If you come across a very damaged screw-head - or a very stubborn screw, first start with the WD40, putting a spray of it around the screw and under the head. Make sure to let it work for a few minutes, then wipe away as much as you can - so your tool can get good purchase.
High-Torque electric screwdrivers can sometimes take screws out with ease that you had no hope of doing yourself - they are a very advisable tool here. Try one before taking further measures if you have one to hand.
One option is to use an el-cheapo alan key (the kind you get free with self-assembly furniture) - or your worst screwdriver, and some superglue to attach a nice lever to undo the faulty screw.
I suggest you throw these away. This technique may also be used on “tamper-proof” screws when salvaging.
Failing this, you may have to go to the more extreme tactic of actually drilling out the screw. Get yourself a screw removal drill bit - they tend to be slightly harder wearing, and have a different blade angle from a general purpose bit. Then drill out the center of the screw.