If the head is sunk into the deck-board and you can get no purchase on it, I’d drill the head out using a metal bit slightly wider than the shaft so that as you drill in, the head shears off. Once the board has been removed, effectively ripped off this remaining screw, you can get purchase using pliers.
If you’ve got the screw part-way out but buggered the head, pliers will grip.
If you’ve buggered the head, you might get a straight hand screwdriver in to loosen the screw enough to get pliers or mole-grips onto it for removal.
Hope this helps?
Mark
Turtle Productions