This Secret Achievement is for accumulating 8 season championships in Sprint Cup. Important to note that Sprint Cup is the top level of racing in this game, so the achievement does not include championships in Nationwide Cars or Craftsman Trucks when calculating the 8 season championships in Sprint Cup. Also Sprint Driver Challenges have nothing to do with this achievement.
Most efficient way to do this is going to be racing in your Career mode so you are multitasking toward your championship achievements and the other secret achievement for 200 career race wins.
Best way to save as much time as possible is to race the shortest distance and easiest difficulty. You can change these in Settings. Then I would suggest you actually skip practice and jump to qualifying. When you qualify your first lap, check the standing on the bottom right of the screen. If it says you are 1/43 (you are the fastest qualifier) then you can quit qualifying. This way you don't have to drive the second lap, but it retains your qualifying spot. Go right into the race. Most races are going to be 15 miles. Each event is going to take you around 9-11 minutes.
Another tip is to skip the road courses and short ovals. These take longer because you drive a slower average speed. If you are winning most of the speedways and super speedways, then you can afford to skip these and still make the Chase/win the Championship. Just monitor the standings. To skip a race, you have to go into the event, then just select "Race" but as soon as you are in the race go to menu and Quit.
Finally, turn off auto-save when you are going for Championships. If you have a bad set of results you will want to back out to the XBox dashboard and go back to a save file from earlier. I learned this lesson the hard way when I had auto save ON and lost a Nationwide series championship by 7 points after finishing the last race 18th, and autosave locked in the bad result.
This achievement is a long haul but will probably be the last one you need to finish the 1000 GS on NASCAR 09, and it's a relief to hear the achievement pop when you get there.