FINAL FANTASY XII THE ZODIAC AGE

A chain is built by consecutively killing enemies of the same species. Killing an enemy of a different species, interacting with a save crystal or entering a town (any area where you only have Vaan walking around) will reset the chain.

Whilst you can theoretically get this in almost any location by avoiding enemies and using zone transitions to respawn enemies the best location to do this early on is in the Lhusu Mines. On your first trip through the mines the areas you can access will be limited and you'll only encounter Skeletons/Skull Defenders and Steelings. For this achievement you want to ignore the Steelings and focus on the Skeletons/Skull Defenders.

The zones you'll use are Shaft Entry, Oltam Span, Transitway 1, Shunia Twinspan and Site 2. The first three areas spawn both enemy types whilst the last two only spawn Skeletons/Skull Defenders. So make your way through the first three areas whilst avoiding any Steelings that spawn until you reach Shunia Twinspan. Once there if you remain on the upper level enough enemies should continue to spawn to reach a 50 chain. If you progress into Site 2 there is a boss fight but this doesn't break the chain.