This solution will net you 900 gamerscore in less than 20 minutes (not counting set-up time, so maybe 30 minutes with setup time). After that, it details how to get Pathological Liar. I tested this solution on my wife's account and had 900/1000 in 18 minutes. She also unlocked Pathological Liar as soon as I launched the game, presumably because I already completed it and the game checks the system save for questions seen not individual gamertags' saves. Weird.
Ok a little prep goes a long way in this game. We can get 900/1000gs in one round by ourselves if we do a little pre-game setting up. If you don't already have it, install the Firefox web browser on your laptop/tablet (possibly even cell phone but personally I wouldn't want to manage 8 windows on a cell phone lol).
Next, download the Multifox browser add-on for Firefox. Disclaimer: DO THIS AT YOUR OWN RISK. If you do it, I would suggest uninstalling it as soon as you finish the game as it's an unlicensed add-on. It can be found here:
https://addons.mozilla.org/de/firefox/addon/multifox/ Click Zu Firefox Hinzufug to download it.
Your alternative is to use multiple cell phones/laptops/tablets to play the game, and maybe recruit 1 or 2 friends to help you, or find 7 people and play a legit 8 player game. You can also use multiple different web browsers (Internet Explorer, Firefox, Google Chrome, etc) on one device to count as multiple players.
If using my Firefox method: Open Firefox on your device. In the top-right corner you should see a new little tab that if you mouse over it shows 'Multifox'. Click this to open up a new browser that Fibbage will recognize as a separate user. In this manner you can have 8 browsers open, to allow you to play as all 8 players. Head to www.fibbage.com on all 8 browsers (or 8 devices, or 8 players if you're playing with other people).
Next, launch Fibbage on your Xbox One and put the game room code on your screen into each browser/device. Once all in, select 'Everyone's In!' to start the game.
On the first question, put K in as your answer for player 1 and 2, and L as your answer for players 3-8.
You should unlock: Type-OH! and Lie Jinx
The Lie Jinx achievement in Fibbage worth 103 pointsEnter the same thing as another player Once everyone has entered their 'lie', have at least one player among players 3-8 choose your lie (which would be K if you entered what I said to as we're pretending you're player 1) to unlock: In the Bluff.
On the screen where all the players can 'like' a lie, have players 3-8 choose your lie (player 1's lie, the letter K), which if you're following this should be 'K'. It doesn't matter if players 1 and 2 'like' anything. After this, you will need to 'like' player 1's answer on at least one more question by any player 2-8 so that Players 1 and 2 aren't tied at the end. This will net you Miss Congeniality at the end of the game so keep that in mind.
On the next question, click the 'Lie for Me!' button on any of the 8 players and pick one of the lie options and you'll unlock: It's Okay to Ask for Help.
Now on the screen where everyone chooses what they think is the truth, have players 2-8 choose Player 1's lie from the above step. This will unlock: Master Baiter.
The Master Baiter achievement in Fibbage worth 110 pointsFool every other player with a Lie in a 3+ player game On question 3 (or the next question if you happened to ignore/skip one), hit the guide button to pause the game and make sure you have the right answer to the question. Enter that correct answer for Player 1. This will unlock: George Washington
It will force you to choose a lie after you answer with the truth here. Have the other 7 players lie with a random letter or whatever.
Then on the screen where you choose what you think is the truth, have all 8 players pick the correct answer (the one player one gave before being forced to lie). This will unlock: Can't Be Fooled Again
From here you can keep answering questions or just let the game idle to the end. Just make sure to 'like' Player 1's answer at least one additional time to the one earlier so that Player 1 & 2 won't be tied.. Either way, at the end of the game you'll unlock: Eight Ball, and if you did what I said for the likes you'll unlock: Miss Congeniality.
The Eight Ball achievement in Fibbage worth 159 pointsPlay a full game with 8 players Your final achievement will be Pathological Liar. You have two options for this. Just keep playing matches (either with other people or by yourself controlling two players via two browser windows or two devices). If you do this you'll want to keep track of which categories you pick, and always pick a category you've never picked before if possible. It will take several hours to complete the game doing it this way.
The other option is just to keep starting games and doing something else while the game idles through questions. This takes much longer to run through every question in the game, but the plus side is you can just have your phone/tablet handy and keep starting new games with the 'same players' option each time a game ends. That way you can play something else (your 360, Windows Phone, or Win8 games, or watch TV/movie, clean your house, you get the point) while the game idles the last achievement for you. It may take a long time, but eventually it'll come. It took me around 8 or 9 hours of letting the game idle, but I forgot about it several times and didn't always start a new game right when the previous one finished. Whether you play legit or let it idle, eventually you'll see all questions.
Once you've seen every question in the game, you'll get: Pathological Liar.
Congratz on a fun (if you played with others and not with 8 browsers like me lol) and easy 1000gs!