I use a 1/4" drive ratchet. A driver bit fits perfectly into one of the sockets, 6 mm I think.
Maybe 5 mm. This will give you enough leverage to hold the screw head without stripping.
If it's too late for that, then I guess you'd better get one of those "screwdrivers" that you whack with a hammer.
Once you get them out then you'll want to replace them with something with a better head. Hex heads or Allen heads are good. If you have trouble finding them, then Phillips heads are acceptable. You'll only need to replace every now and then when stripping starts.
Those bolts are either 5 mm or 6 mm. I don't remember off-hand. But you'll be able to figure that out.
Good luck.
