Forza Fifty Achievement
Reach Driver Level 50.
The leveling achievements should all happen naturally as your work on other achievements in the game. XP is awarded based on the length of the race rather than the difficulty, so changing the assists and Drivatar difficulty does not impact how fast you level up.
To determine how XP is needed for any given level, the formula is (Level x 14,000) - 10,000. For level fifty, you therefore need (50 x 14,000) - 10,000 = 690,000 XP.
Note that the achievement actually requires you to complete a race while at this driver level, so it will unlock at the end of the next race after you reach level fifty.