This achievement is known to glitch on some people who have spectated 20 games. There have been reports of it not unlocking. If this happens, do not bother watching more matches. Instead do this:
1. Simply go to the Xbox Dashboard
2. Clear your cache by highlighting it under Storage and pressing Y.
3. Backup your Blazblue CT save if needed (move it to cloud or an external save device)
4. Start up the game and start a new save
Afterward, watch another 20 games in spectator mode in a player match and the achievement should unlock.