This is a guide by nicobest573 from PlaystationTrophies.org.
Original forum thread: http://www.playstationtrophies.org/forum/tekken-6-a/30760-ga...
I have followed this guide and it is the easiest and fastest way to earn the achievement! I have just earned the Gallery achievement by doing this. Even the prologues unlock.
"Here is what I have done to get the trophy under a couple hours.
- First complete Scenario Mode.
- Play [Central Subway Line] (the stage with Nina as the boss) and as soon as the level begins, jump off the train (run towards the edge and press A). Repeat that over and over until you unlock the trophy.
Because the loading time is much faster when retrying this stage, and because it's really quick to just jump off the train, this is the fastest (and yet easiet) way I have found to get the trophy.
If you need some explanation as to why this works, know that once the Scenario Mode is completed, the game will unlock endings as you play. Unlocking of those is not random however. I found that they unlock in the same order as you unlock the characters in Scenario Mode. More importantly, I found that the game counts how many matches you play and awards you with an ending every 25 matches. That means that if you go play some Arcade Mode, every 25th match you play will get you an ending unlocked. Now, if you play Scenario Mode, you only need to play 6 levels (in most of the cases; sometimes you need a bit more, when unlocking dual endings such as Jin's) to unlock an ending (obviously because a Scenario level takes longer to complete than a regular match). The key though is that the game does not care if you win or lose, as you long as you play. So keep losing a Scenario level as fast as you can and you'll get the trophy in no-time."