After you take control of the Mantis the first time, a cutscene will occur. Right by the tentacle near the end of the game, you'll be back in control of the Mantis and S.A.M. will tell you that your shields are down. Fire around the room and after it's clear continue on towards the objective firing at everything. As you continue on, pods will be littered throughout the cave. Mindlessly firing towards them and shooting around in every room should net you more than enough pods to get you the achievement. The bar will pop up every 10 pods destroyed, so you'll know if you're doing it right.