The Outer Worlds

Lucky's body is on Gorgon believe it or not. It can be found in the bottom right area of the map in Gorgon's Canyon. Its inside a marauder base call "Marauder Stronghold" off to the side in a cave being devoured by a candid.

Here is a video guide on where to find him if you are still having trouble:

The achievement pops when you discover Lucky Montoya's body. From Khaleel Chakroun who runs the General Store next to the Gorgon landing pad, you learn of Charles from Accounting. He hangs out at the Marauder Camp in the southeastern corner of the Gorgon Canyon map.

From Khaleel's Survival Goods, follow the darkest orange terrain on the map in roughly a southeast by east direction. Be mindful of the enemies along the entire route to the Marauder Camp. Turn left just before the OCI Lower Exterior. The OCI Lower Exterior will show up on your map as a fast travel point once you approach it. Referring to your map, you'll see the path veers downward into the Worker Domiciles. If you haven't been there yet, you will discover it, earn XP, and it will show up as a location on the map.

Continue down a short ways until you reach an encampment. This is the Marauder Camp. Defeat the marauders with special care towards Charles from Accounting. Don't forget to loot his body because he carries "Special Delivery," one of three science weapons required for:

After you defeat the marauders, refer to your map, and head to the lower right boundary of the encampment. Enter the passageway you find there. You'll see a Canid investigating a body. Kill the Canid and approach to discover Lucky Montoya's body and pop the achievement. Next to him is an audio log, "Lucky's Notes: Smoke and Mirrors." Pick it up. It is one of the audio logs required for:

The following is a video that shows Montoya's location on the map followed by the discovery of his body and picking up the audio log: