1. Head to North San D'oria and speak to Miaux. You should get a cutscene.
2. Go to East Altepa to around the middle of the map and click on the ??? near a ruins. (I marked it on my map, but forgot to write down the exact coordinates. I'll post them later.) You'll then fight an NM. It was moderately hard, so be sure to have a full PT of 50+ or so.
3. Click on the ??? again once you finish the battle.
4. Return to Miaux to complete the quest and get your AF1 weapon!