How can I "train" my JUNIOR boss to stop wasting my time so I can focus for my SENIOR VIP*** boss?

Corporate law firm. I have 2 bosses: 1 top-flight VIP*** & 1 very junior & new to firm. The junior boss is hopeless disorganised inexperienced wet behind the ears "L" plate timewaster, muddles & transposes figures - like client's bank account details! - & words in her dictations, who can't give clear practical instructions so I waste disproportionate mega-man-hours on her small fry stuff when I should be doing a LOT LOT more for my VIP boss. She, the small fry, doesn't seem to take it in that I have to be at my desk and "available" for him, the VIP boss, his calls, his emails, organising & attending to him, & instead she has me running "urgent" errands & doing "urgent" bulk photocopying etc. (at the last minute so it's too late to send it to the copying dept). She's a damn mess! How can I train her to use me properly? She is not my priority but still she's my "boss", thick though she is, & how can I train her? People are saying it's up to me to train her! She has the skin of a rhino!