Other guide missed multiple important clarifications, so let me try, 5 years late :)
Most important is to choose the right Shadow Survival. There are two ways to do it, but one is glitched and will NOT unlock achievement.
CORRECT: Single Player > Fight > Shadow Lab> Select your shadow > Shadow Survival
INCORRECT - DO NOT USE: Single Player > Fight > Survival > Shadow Survival
- You must defeat 25 shadows total. Thankfully, these do not have to be consecutive, or in the same session
- Your shadow will not record any wins or loses, so don't worry about this
- If you get a download shadow error, say yes once and no the second time seems to fix it most times, YMMV
- Whether you save your recording or not is irrelevant for this achievement. If you save, you can stack with 'higher learning', or can skip saving if you want to keep your shadow as is, or save a few clicks