This is a story related achievement and
CAN'T BE MISSEDSPOILERS*** Spoiler - click to reveal ***Right at the very start of the game (Disc 1) you will be tasked with accompanying Quistis to the Fire Cavern as part of pre-requisite to becoming a SEED. The player will meet with Quistis at the entryway of the garden and after some tedious dialogue you will venture east of Balamb to the Fire Cavern for some more tedious dialogue. There are two guards outside of the entrance to the Fire Cavern who will give you an option of setting a timer for your adventure. Choose whatever timer suits you (I generally just do 30mins which is an absolute breeze) then enter and follow the path straight through to the end where you will encounter the GF Ifrit. In order to UNLOCK ifrit you simply just have to beat him. He is weak to ice so hitting him with Shiva makes this fight incredibly simple. Upon beating him the achievement should unlock!