Unlock Guardian Force TonberryThe Tonberry GF is obtained in the Centra Ruins area. This is annoying to accomplish because you first have to defeat 20 regular Tonberry enemies before the Tonberry King attacks you, and there is a 20-minute timer going in this area. Simply leave and return to reset the timer, and your count toward the 20 kills will not be reset.
Tonberry enemies have a move called Everyone’s Grudge which does 20x the number of enemies that party member has killed. This shouldn't hurt anyone but Squall too badly. However, if you let them take two steps forward before attacking, they will not counter with this move. Defeat 20 regular enemies and the king will show up in the same battle where you kill the 20th. His attack pattern is the same as the basic enemies. Defeat him to unlock the achievement.