Valkyria Revolution

Ragnite are essentially spells that will be dropped by enemies as you defeat them. There is no guarantee you'll get any during a mission, but it's likely you'll at least get a few, sometimes quite a few. The level of ragnite is based on the level of the mission you're doing. Rank 30 ragnite drops readily during Level 60 missions (though it doesn't matter the type of mission). You'll be grinding out a bunch of Level 60 Free missions after you finish the story, so you'll get plenty of these along the way, and you only need one for the achievement.