Actually, the same holds true the other way - Diggersby can switch in onto a predicted Shadow Ball from Aegislash and can proceed to beat it with EQ, making it an excellent Aegislash check.However diggersby's choice sets are where the problems arise. Let's face it, Normal typing is just not very good this meta. Gengar, Skarmory, Aegislash, and many many more Pokemon are able to play around a choiced Diggersby. This is usually where your "prediction" skills come in however it depends on you and your opponent.
Actually, I've been trying out the Scarf set and it is very good - the sheer surprise factor is great, U-Turn makes it a great pivot(especially as it can hit a Lati twin switch in quite hard), and it can then late-game clean with Return or EQ.The best Diggersby set by far the LO Swords Dance set
I agree with the proposal to move Diggersby to A-, as it is an excellent wallbreaker and a good pivot too. That's not to say it doesn't have shortcomings, but I feel A- fits Diggersby better right now.