You drive the Mantis on level 19. There will be two sections where you control the Mantis: one where you will be killing cultists and bugs and the second after a cutscene. The pods are in the second section. The Mantis is a very powerful vehicle and there are around 250+ pods, so this achievement is really easy to get. In a couple of minutes of controlling the Mantis you should get very close to the 100 mark. Press to spot pods easier. To check your progress press > Extras > Player Stats > >Achievements and then find the achievement and it will have your current progress.