Use the largest JIS Screw driver you can fit and hope for the best. Otherwise, a dremel cutting wheel and a slot is about the only option after drilling off the head. Using standard phillips head drivers = rounded out JIS screws.
If you don't have JIS screwdrivers, get a set. There is a decent affordable set on Amazon. I use the 2 largest sizes in that kit the most, and bought 2 sets to have one for an on the road tool kit.
https://www.amazon.com/Hozan-JIS-4-J...880026860&th=1