Failing there being a proper tool, will a box spanner or socket fit over and allow the nut to be turned?
Must say that the usual way of dealing with something like this is as Madrab says. Tap it round. If you put the screwdriver straight in the slot, and tap the side, of the shaft as near to the end as possible, rather than using it like a chisel, you may get away without damaging the nut.
I've made tools for this sort of nut in the past, either a steel tube with a couple of "nibs" filed on the end, or a couple of bits of flat steel, bent over to suit, filed if needed, and welded bolted or clamped together.