You can actually get this trophy through natural progression of the game, as odds are you will get at least 30 headshots, even if you are not that good.
If you wish to get this earlier though, in the beginning of the game you will meet Jacob. This is basically a tutorial on manually using biotic/tech powers. The game wants you to press R2 and bring up the power wheel. Don't do this, and instead keep headshotting the mechs as they continually come out of the door.
There should be spare ammo in the area if you run out. Once you get the trophy, use Jacob's Pull, and you can move on. Remember, these must on humanoid targets, which means they must resemble a human's physique.
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