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 the first battle it will ask you if you want to stay and fight or retreat. Select stay and fight to fight the next wave. Each wave the enemies will increase a couple levels, this isn't to much of a problem until level 5. The enemies you face on level 5 will be anywhere from Lv 19 - Lv 23 If you are playing on Cadet difficulty. Play this on Cadet, because on officer difficulty the enemies on Officer difficulty tend to be in the mid 30's on wave 5 making this achievement harder than it needs to be.
(Thanks to Neeker75 for stating that he fought Lv 19 enemies on wave 5 on Cadet. I started on officer difficulty making this achievement harder than it should have been.)