Forza Motorsport 5

This achievement took me a while to get, but this is the easiest and fastest way to level up in Forza 5 -
Go to Multiplayer and press Y to create a private match
Pick a car like the #1 Audi Team Joest R18 e-tron Quattro ( a fast car with good handling)
Then go to change options and then Game Options, then pick these -
Game Type - Circuit Race
Environment - Indianapolis
Track - Indianapolis Oval
Max Players - 16
Max Drivatars - 15 (That is what I prefer but you can do any amount as long as its more than 10)
Laps - 20 ( You can do more but 20 laps takes about 15 - 20 minutes to complete depending on the car)
Every lap you do is an extra 1,000 xp so 50 laps is 51,000 and 20 is 21,000
The rest of the settings are whatever you want to put

It does take a while to get to 150 but if you do this around 5-10 times a day you should get it pretty quick
Also if you have Netflix you can snap it so you can play and watch just put a rubber band around RT and steer
Good luck on leveling up!