The Butterfly Idol achievement is worth 15G and is unlocked by swimming the 100m Butterfly in under 50.40 seconds.

I suggest attempting this in training, getting a really good start can be the key to beating the time. Therefore if you false start the race will automatically restart, and then you dont risk disqualification from a competition. If you get the perfect start your speed bar will be maxed out from the start. This will save you the added energy to get it to this point

I would also recommend using the analog sticks. It takes less energy to keep your swimmer at maximum speed, downside is it takes more coordination. Its not a problem if you prefer using the combination of [ Register or Signin to view external links. ] + [ Register or Signin to view external links. ] , it just takes a lot more effort to maintain high speed.

If you feel yourself getting tired press pause, (or Xbox button) it will give yourself a break to get your energy back. Make sure you time your kick turn well, if you can get a perfect it will keep your swimmers speed at maximum, which will help you if your losing your speed towards the 50 metre mark.

Should you still have trouble beating the time, try using slow-mo in Olympic mode to maintain top speed.