How to remove wood filler to get to the screws
First off, make sure there aren't nuts and washers on the other side of that wobbly leg that you could just tighten up.
Drill a small hole in the filler/glue down to the screw and then cut away the rest with a utility knife. If you have a Dremel or other spin tool, use a bottoming bit and clean out the area.
You might find that the screw holes were just worn bigger. In that case, you might want to drill right through the leg and get some longer bolts, washers and nuts and re assemble rather than just tightening them up or getting larger screws. That way you won't run into the wobbly leg again.