Sabot Fox is right on the money, but I wanted to go a little more in depth.
50 consecutive wins without using the Retry option lands you this achievement. The Retry option appears a couple of ways:
1. If you die.
2. If your Mog Clock is Green or Yellow and you want to select the Retry option mid-battle (you cannot select Retry if Mog Clock hits Red).
I highly recommend getting this in the first area of the game (New Bodhum AF 003). Reason being is that many of the monsters here are one-turn enemies (meaning you can win battles in one turn).
Another reason is that Potions and Phoenix Downs drop a LOT in this first area. I don't know how many Potions and Phoenix Downs we start with in the game, but between killing monsters and treasure chests, I had 92 Potions and 13 Phoenix Downs by the time I moved on to the next area and I only fought 3 or 4 battles above the required 50 for this achievement. This saves a TON of cash early. I also ended up with 8 Iron Bangles. So this will save you that much needed gil early on.
Here are the costs of these items if you bought them at Chocolina:
Potion: 160 gil
Phoenix Down: 1600 gil
Iron Bangle: 300 gil.
Good luck and enjoy FFXIII-2!