30 Kills: Arm Micro Trophy in Uncharted 3: Drake's Deception

The Arm Micro is quite a common side-arm weapon and 30 kills should come naturally if you use it enough instead of your rifle. You first pick one up in Chapter 6 as you emerge from the well into the front of the estate and is dropped by many of the enemies. There are a couple of good farming spots should you need to churn out kills:

Chapter 6 - The Château

After the grenade tutorial you will emerge in the courtyard with Sully taking fire below. There are a couple of Arm Micro guns below on some crates as well as many enemies that carry one so ensure you pick them up to stock up on ammo when running low. Leave one guy alive and restart checkpoint as required.

Chapter 8 - Citadel

At the beginning of this level you traverse up the building to a window ledge and hop through a window opening taking cover from enemies below. Shoot one guy on any adjacent platform and the nearest guy jumps up from below to come for you, take him down and pick up his Arm Micro. With this, take out the remaining guys except for one, then restart checkpoint as required.