I have been able to remove stripped screws by leverage applied to the screw/bolt head and then using the correct tool to back out the fastener. The leverage comes from a tool with a fairly sharp edge that is wedged underneath the fastener's head then use the correct tool to remove the fastener. Tools to consider are gasket scrapers, wood chisels, screwdrivers, a knife (obviously one in less than good condition or an inexpensive one - I have only used a knife one time when stranded on the road and desperate), etc. (you may need to be imaginative depending on the extent of your tool collection). The leverage method does not work in all cases, but has worked often for me as a professional technician/mechanic.
Or, you can try a pair of pliers on the head and pull up at the same time you loosen the screw/bolt.
I own a 300D and not familiar with the fastener on your 280 - so I cannot give specific info about your engine. Perhaps another member has experience with your problem (for example does this fastener have a backing nut?).
Hope this helps.