NieR

This will have spoilers, so I hope you've already beaten the game once.

This achievement is relatively simple, it's essentially just speedrunning the game. I'd recommend leaving this achievement for last, because you can't get it after you beat the game(You need to start a file from Part 1 and finish Part 2 with that same file under 15 hours) and you'll miss out on a bunch of dialogue if you try to get this off the bat.
For starters, play the game through normally. You'll know the basics of the game and the story, but if you don't, here's some tips:
Play on Normal. You can also do it on Easy, but where's the fun in that? Still, it's your choice. Hard mode is a bit of a waste, everything becomes a damage sponge and takes way too long to kill, especially in Part 1.
Roll. Roll everywhere. Never ever stop rolling. It's faster than walking and gives you invincibility frames so you can dodge through those red ball projectiles and occasionally boss attacks.
Dark Hand is your friend. During Part 1 you do terrible damage, but that's where Dark Hand comes in. It can kill or terribly wound any enemy in one hit, your MP recharges quite fast, and it's unlocked very early in the game. Bosses are also a breeze when you have this attack.
Don't forget Dark Lance and Dark Blast. Dark Lance can help you during parts like the boss in Facade(The temple) and the Aerie(Part 2), and Dark Blast can help you kill flying enemies that come in droves(Like those flying pests in the Junk Heap).
Ignore Defense. It's a waste of a skill slot, it's terrible, and Evade outclasses it in every single way. Replace it with Dark Hand or whatever you prefer.
Spears are your best weapon. They not only break the game(Phoenix Spear), but they also allow you to spear dash. Doing that combined with rolling gives you such a speed increase that there's no reason to walk.
Be weary of your health. Boss battles with stuff attacking you aside from the boss can get really hectic(Especially in Part 1 near the end with the regenerating boss)
Ignore everything. Sure, that glowing item might have rare stuff, or that sheep really needs to die, but you're wasting time, especially when you don't need to be. You can kill shades if you want, but most of the time don't bother.
Don't do quests. If you plan on doing all the quests for Part 1, you better make sure you skip every single line of dialogue and are amazing at fishing, because it takes around 10-20 hours just to finish those.
If you need to pause, go and save. When you pause or Guide Button, time is still ticking. So if you need to go do something(Like go to the bathroom or go somewhere) I'd recommend to save and just load that save when you're ready to do it again.
Skip everything. If it's a cutscene, spam Start. If it's written dialogue you can't skip, spam Start, A, and B. The only time you shouldn't do this is during the Forest of Myth key part.

Now that we're ready, part 1 is extremely easy for the most part. The only thing I'd recommend is do the Fragile Package quest on your way to the Aerie for the free 1500 along with the Boar quest as soon as possible. Boar lets you go so much faster than rolling, and Fragile Package is easy because you just need to run and none of the Shades are fast enough to catch and hit you.
For the Forest of Myth, you only need to talk to the Mayor and can ignore the other two people. Here's the answers:
A secret.
Sunshine
A man.

Next, when Popola asks you to get that one moss from the Southern gate and the boss breaks though, damage him enough so he moves ahead to that little...bridge(I'm not sure what to call it, it's that giant grey thing you pass under with stairs on the left that take you to a hill and to the boat). If you want, spend around 20 minutes sitting here. The enemies constantly respawn and drop a bunch of Health Salves and words, and your Dark Hand will nearly kill them in one hit(It WILL kill in one hit if it smashes the Shade into a wall).

For part 2, everything is straightforward. The first thing you should do is sell EVERYTHING in your inventory and if you have 32,000 gold you should go to Facade, talk to the blacksmith, and buy the Phoenix Spear. It is the single best weapon in the game.
Next, I'd say go to the Forest of Myth. Answers are like this:
What is the color of the kid's eyes?
How many people did the soldier say died?
Any of the Yonah answers will do.
Do the Junk Heap and save Facade for last(Unless you just want to get that out of the way).
Everything else should be insanely easy thanks to the Phoenix Spear.

Just a little thing to note, I managed to beat Part 1 in 3:38:54. I took like a 20 minute break and grinded for 10 minutes at the Part 1 boss portion I mentioned, but otherwise it's been rolling everywhere. I just beat Part 2 with a total time of 5:49:29, it was extremely easy, although P-33 almost killed me once.

All in all, it's a very simple run, shouldn't take more than 8 hours maximum, and is easy to do.

This achievement is annoying, because it must be done in one playthrough from the start of the game to Ending A. Repeated playthroughs just continue adding on to your played time, so if you didnt get to ending A in 15 hrs, you gotta try it again from scratch or an alternate past save that you think you could rush to the end in time from (this is what I did- I didn't realize you had to do 15hr flat in one straight run until after I had done a ton of sidequests- luckily I had an alternate save from about halfway through the game from which I was able to barely rush from to the end and get this.)

Here is the plan of action recommended from PerFecK's Achievement Roadmap + Ending guide ( http://www.xbox360achievements.org/forum/showthread.php?t=22... ) which are both immensely helful here:

Ending A: Plan to beat the game in under 15 hours to earn Lightspeed Fighter. It is recommended that you play on the Normal difficulty (game allows you to change difficulty at any point) or lower to accomplish this. To give you a general idea, I was able to beat the game in a speed run in 7 hours and 30 minutes, which included skipping all text and cutscenes (and I knew exactly what I was doing). Since its the first time you are playing the game, enjoy the story and dont worry about skipping cutscenes, but complete a minimal amount of quests and always follow where the red x is telling you to go next. **However, to enable gardening and more efficient gardening, you must complete 2 quests, so do quests "Shopping List" and "A Return to Shopping" given from the Village's florist ASAP because they can be missed later in the story!** (Rastal's note: This is incorrect. You may skip these quests as well if you want because you can buy a cultivation guide which is not expensive from this npc in part II to enable the garden.) If you follow these steps and play on Easy/Normal, you shouldn't have a hard time making 15 hours. At some point early in the game (because after a certain event sheep will no longer be found) - save your game and get The Sheep Whisperer. Then load your game directly after to ensure you keep a low playtime. At the end of this run, you should have grabbed Call Her Back, Material Hunter, All Aboarded!, and all of the story related achievements.

My recommendation, based off of what I did, was to have two saves running in parallel. One for playing and enjoying the game, and the second for the speed run.

Save spots #1 and #2 are for your main game. Do your exploring, questing, and general playthrough here. I tend to 'roll' my saves, alternating between spots so I can back up to the previous one just in case.

Save spot #3 is the speed run save. This is for your Ending A playthrough within 15 hours.

Play through on the first set, going at your own pace. Upon going through the core storyline and beating a boss, save it, then load up the second 'speed-run' save. Use what you learned during the primary quest line to quickly get to, then dispatch, the main boss. Skip cutscenes everywhere to ensure they don't count against your time. Save after completing, and exit back to your main save to once more enjoy the story.

In this manner, I could get the speed run without sacrificing the RPG-ness of talking to people, doing side quests, watching cut-scenes, and just plain enjoying the game. Don't expect to get many achievements on this second save; it's just to finish the game ASAP, while still having the sections fresh in your mind. You may get some of the boss speed-runs too, but those may require upgraded weapons and abilities that come from New Game+.