This can be done anytime after chapter 1. These battles are the random encounter's you face on the Overworld map (When your outside Akademia). After you win your first battle, you will be awarded the achievement. You can pair this with the Total Domination achievement for winning 5 of these battles in a row. Be warned this achievement is much harder than Roaming Terror.
If you need help winning 5 in a row you can reference the guide I created for tips to beating the Tough wave 5 underneath.