You must finish a level with five Sentry Guns in your inventory. You can check how many you have at anytime by accessing an Intex Terminal. Find Sentry Guns throughout each story level as well as buy them from Intex Terminals whenever you see them in stock. Sentry Guns are available in Terminals for $3,000 credits usually when there are rooms nearby with Power Nodes.
I was only able to find 3 Sentry Guns throughout my story playthrough, so I'm pretty certain you must get the rest of the Guns from the Terminals. You should be able to buy at least 2-3 from each level, so purchase them whenever you have credits to spare. The trophy will unlock during the results screen at the end of a level.