There are plenty of games you can choose to get this but, if you want the best and quickest method to get it quickly, I shall explain...
Choose the 'Slapping Station' game and follow the directions. You do not need to move left and right at all but it does help to make a forward slapping motion to hit the Rabbids. Hold your arm out straight, but at a 45 degree angle to your body to hit the low ones, 90 degrees to hit the middle ones and you have to jump and slap to get the high ones.
You'll want to 'slap' as the rabbids as they get level with your man on the screen. If you stay in the middle, it's easier to see what's coming. Try not to hit the cows or miss any rabbids. At the end of the game, loads of Rabbids will stream past; just hold out your arm(s) at the right hight and you'll hit them. You can get over 100 in this bit alone!
Once you become used to the game, you can hit up to approx 170 rabbids on each turn. It took me about an hour to get from 1024 to 5120 and then about another hour forty to get from 5012 to 10240.
This will get you these 2 achievements also: -
Here's a video made by MilleniumGamingX if you want to see it in action.