First off, to be able to fly around in Assistance OFF mode in the campaign, you must have at least completed mission 6. Once you've done that, spend a bunch of time flying in OFF mode.
Getting used to OFF mode may prove tricky for some. The default way OFF mode handles causes your plane to go left or right according to your view when moving LS left or right. Pressing Up and Down will cause your plane to dive or climb. All needed maneuvering to get the plane to move in those directions is handled automatically. You can switch it to Expert mode in the options menu. This will cause all plane controls to remain the same, left and right rolls while up and down lowers and raises the nose.
A word of cation about OFF mode, don't use it when engaging ground targets on uneven terrain or in urban battlefields. The constantly changing camera angle is likely to send you straight into a cliff or building.
As of right now, I am uncertain as to whether or not time spent in OFF mode during Versus mode counts. However, one of the campaign challenges is for flying in OFF mode for an hour. Because of the relationship between several other of the achievements and challenges, I have to assume it doesn't.