Tony Hawks Project 8

Create a match with the game high score challenge to 10,000 you should complete a match every 5-10 seconds and have one hundred games in about 45 minutes

This achievement can be done in Quick Match: Player (Does not have to be ranked). The best way to get this are the following settings:

GameType: Score Challenge
Scoring: Points
End: 10,000 pts
Time: 30 Seconds
Map: (doesn't matter, but limit it, to Suburbia only for example)

No matter what, the games will take 30 seconds MAX and if someone runs into someone else's wall maybe 10 seconds. I was 0-100 in about 40 minutes. This works for the 1 game, 50 game and 100 game achievement.

***We could never figure it out, but there is a catch to this. We played roughly 100 matches, and the only person to get it was the host. We switched up Hosts, and in the next game someone got 50 games. We let them continue to host and soon had 100, and the other 2 boosters were just to 50. I appears it is 1 game as a host, else it is 1 game per win. I would suggest if you get 2 people...have the host always try to lose. If you figure anything else out, please comment below.***

To add, you have to score a trick or host to get a talleymark, so the Walls game doesn't work as the best option in a party of more than 2. Use the updated Score Challenge outlined above. As noted in the comments too, as long as you start the server with two players, you can continue playing by yourself as the host without any problems all the way to 100.

This achievement will take no more then 45 minutes.

The fastest way to get this achievement is to host a game of Walls (last man standing) and invite a friend. When your friend joins, start the game and have them quit the match. Once this happens you receive an automatic win and you can keep on restarting the game until the achievement unlocks.

*Note: this method can be done with a random person but it takes longer to try and get the random player to quit the match.*

First create an xbox live custom match. change the maximum players to 2. the in-game settings need to be a high score challenge of 10,000 as this can be achieved in a single quick combo. There is no need for any other players to be in the lobby and you can just keep re-launching the combo challenge after each time. Each time the combo of 10k is reached this counts as a game, just rinse and repeat 99 more times and the achievement is yours, can be done in about 30 minutes or so and you will also get the 50 online games and the first game online achievements in the process.
Hope this helps :)