This is in Chapter 6 in Junon, obviously. The game will advise you of your objective to prevent any enemies from reaching the objective. Run down the street and stand near the purple line and just run into the enemy groups as they come to engage. I don't think the other enemy groups continue advancing while in combat so don't worry about rushing too much once you're in actual battle.
There are a total of 30 enemies, each group had 2-4 enemies for me and the achievement should pop once all 30 are defeated. Video from PowerPyx for those who'd like it.