You can only get this while in Free Play, after you have at least completed the main story level The Taming of Gollum, which is located in Lothlorien.
After you complete the story level, you will automatically gain access to Gollum. Play the level again in Free Play, and you can get the token for Smeagol, if you have the correct character unlocked.
You can then purchase Smeagol at the cart for 80,000 studs. There is a problem while you are in the open world, as it will not allow you to select both Smeagol and Gollum simultaneously. Go back to The Taming of Gollum level, and select Smeagol as your main character. It will then add Gollum to the Free Play Party, and once the level starts you can defeat Gollum. The trophy will unlock right after.
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