Ah yes, ideally you need a c-spanner, either adjustable or a fixed one of the correct size. Now I shouldn't really tell you this as more lock rings have been damaged by using a hammer and punch to undo them than there are stars in the sky, but you could try a hammer and punch.....gently!
However, slackening the lock ring is half the problem, then you have to undo the adjustable cup. I can't see from the picture which type you have, but it will need either a pin spanner for the holes, or a thin spanner across the flats if there are any.