The first Sentry Bot (Doom 3 Campaign) that will allow you to take this achievement is located in EnPro Entrance. You will have to guide him through Control 1, Unidentified and Maintenance 1 to his end point in Power Core Access. If you play on Recruit difficulty when going for this, you're pretty much guaranteed to get the achievement.
There are over a dozen enemies that attack you, comprised of Imps and Maggots. Make sure to place yourself between the Sentry Bot and the enemies. The Sentry can shoot through/around you well enough to deal damage to all enemies, but the enemies can only hit you. 6-8 Imps and Maggots will spawn in very quick succession in Maintenance 1, so be on your guard.