This achievement requires you to kill 50 enemies of the same genus in a row, without killing any other types of enemies. This can easily be done early on in the game in
Lhusu Mines/ Shunia Twinspan.
The only monsters that spawn in Shunia Twinspan are Skeletons and Skull Defenders. Run back and forth in this area, killing all the enemies until they stop spawning. Once they stop spawning, you will need to move two screen areas away for them to respawn. Make sure you hold RT while doing this so you don't accidentally kill any other enemies along the way. Run down one screen to Transitway 1, right one screen to Oltam Span and then back to Shunia Twinspan. You should be able to get this achievement on your second run through.
I suggest turning x4 Speed Multiplier Mode while doing this. If you are unfamiliar with this setting, press Y to bring up your menu, scroll down to Config, scroll down to Speed Mode Multiplier and select x4. After that, you will be able to toggle between regular speed and 4x by pressing LB during gameplay.
This area is a great place for early EXP, LP and gil farming as well. The enemies drop Bone Fragments that sell for 193 gil each and Dark Stones that sell for 35 gil each.