If you're a lonely schlub like me who doesn't have any friends, this is a way to knock out Shadow Apprentice, Bounty Hunter, and eventually Shadow Ruffian if necessary if no one is challenging/beating your shadow at all from day to day.
1. Create a shadow on both your main account and a dummy account (remember dummy accounts share your Xbox live gold on XB1), playing through the 3 practice matches. DO NOT MOVE AT ALL during the practice matches other than to throw one or two quick punches. Save your shadow data after the practice matches are over.
2. Play ONE survival match against a random opponent's shadow. Again, DO NOT MOVE AT ALL except for one or two quick punches. Save your shadow data. This step probably isn't necessary but I did it so I added it here.
3. On either account, CHALLENGE the other account's shadow and defeat it. DO NOT SAVE YOUR SHADOW DATA after this match. This way your shadow (whether it be your main or the dummy) doesn't get any "better" and will always stand still.
4. Switch accounts to the one that was challenged and defeated, and play the bounty match against the other, defeating it as quickly as you can, but again DO NOT SAVE YOUR SHADOW DATA after the match. Just back out to the Shadow Lab so the win/loss for each account saves to the server.
5. Switch accounts to the other one and defeat the bounty match in the same method. Another reminder, DO NOT SAVE YOUR SHADOW DATA after the match. The idea is to keep getting your bounty wins without 'improving' either account's shadow.
6. Rinse and repeat, switching back and forth and defeating the bounties without saving the shadow data until you have your 25 wins. You'll get Shadow Apprentice in the process. It should take 1.5-2 hours with all the down time/loading time between matches to get Shadow Apprentice and Bounty Hunter. For Shadow Ruffian you're looking at 4x that or roughly 6-8 hours of grinding. Hopefully you still need fight challenges with the characters currently available. Why the hell we can't use the latest fighters so we can work on THEIR challenges is beyond me.
Here's a video tutorial: