Grand Theft Auto IV

To get this achievements easily, all you need to do is whack one guy off of his bike (hold X or B until Jonny swings the bat right back) then sit next to it on your bike and every time the biker gets back on just hit him off again (hold X or B while he is walking back to his bike so that you are ready to hit him off).
Whacking off 69 guys might sound like a lot *sniggers* but it does go surprisingly quickly.

To avoid confusion, please note that this achievement is done in the single player races, not multiplayer.

If you practice this strategy, you can hit several bikers with one swing when they are stuck in front of the firetruck.

On your bike, you can swing your bat left or right using X or B. However, a short swing may not force a biker off the bike completely. In this case, you can hold the X or B buttons for a couple seconds to increase swing power and effectiveness.

To lose the race quickly, in case you mess up, you can jump off your bike and wait for the 10-second timer to expire.

Before you start the first single-player race "Race Park Circuit" in Northwood, Algonquin, drive a little further towards the highway near the railway tracks. You should see a small fire station, and a firetruck should spawn there. Steal the firetruck, and park it so that it blocks the road under the railway tracks. Call 911, and press 3 to ask for another firetruck. Steal the new firetruck, and block the road as well.

Once you have at least two firetrucks blocking the road, start the race, turn around, and immediately drive towards your roadblock. You can steal more firetrucks before the race, but this will attract unwanted police attention. Consider stealing another firetruck during the race so that the police won't intervene. Use the third firetruck to re-enforce your roadblock. Place it in the middle of the road between the two firetrucks. While you park your firetruck, keep your bike near so that you can stop the 10-second end timer.

Wait for the bikers to approach the end of the first lap. When they appear, they will hit the firetruck. Keep in mind that the bikers may bypass the roadblock if they reach the edges or move the firetruck. Swing your bat to prevent them from reaching the lap finish line.

You can die or lose the race as many times as you need. Once you have the achievement, you can reload your saved game before your imperfections affect your beautiful record.

To check your progress, go to the start menu, Stats, Score, and then scroll down to "Number of racers knocked off their bike".