I recently removed a lockring like that. I used my standard lockring wrench, and the biggest flathead screwdriver I could find. The wrench is too big of course- the tooth comes out of the notch before
you can get any leverage on it. What I did was put the side-edge of the screwdriver in one of the other
notches to give the wrench something to push against while the tooth still stayed in.

I know its kind of a rigged way to do it, but it worked for me.