Hi,
What you need to do to earn the achievement :Unique treasure are obtainned by doing Gummi fight in space
and obtain a A rank. This include small fight against wave of enemies or "Boss" fight.
Each fight will only give 1 treasure, once (redoing the same fight is useless once you get your A rank). There is way more than 20 fights (like 30 or 40?) availlable accross the 3 areas, so you can pick the one you prefer.
To get those 20, you can start by facing the enemies you will encounter during your space travel. You can also got the the info menue (start / info / treasure) and see all encounter you alerady have done and your rank. If you did not get an A, you can press
and mark those fights, so you can get there and improve your score.
Getting an A is obviously easier with a better ship, so saving this for after you beat the game with a ship that reach lvl 30/40 and something like Ultima Max could be a good strategy.
How to avoid the tracking BugTracking of the achievement is kind of broken. Even if you get a new treasure, if you press
an look at achievement you will likely not see the achievement progress. Please go back in the game, and go to the page tracking your battle score (start / info / Treasure), and thing will get synchronized again. This will also be needed to actually unlock the achievement after you reach 20 treasures.