If you haven't unlocked this trophy after completing the game, then use Chapter Select to load up Chapter 14 - Cruisin' for a Bruisin' on Very Easy difficulty. Progress until you fall down into the sunken area with the crates and containers shifting back and forth as the ship tilts from side to side.
Kill a couple of guys until a large enemy with a PAK-80 drops down into the area. Kill him and take his weapon, then kill the next several enemies that drop into to the area with it. DO NOT kill the last enemy left up top. Instead, reload the checkpoint and you will start again where the PAK-80 enemy drops into the area. Rinse and repeat until you get your 30 kills and your trophy.
Note: The trophy will unlock as soon as you get the 30th kill.