Steady Hand McDuff can be found in the Lost Bastille. In order to open up his shop, you must find the Dull Ember in the Iron Keep. You must spend about 15,000 souls on upgrading your gear with him (buying items doesn't count). Then talk with him and he will give you a blacksmith hammer and this trophy.

If you are having trouble finding him, there are two ways. Near the tower for the Ruin Sentinel fight, there is a Halberd knight standing at the top of a set of stairs with a barrel. Roll this barrel down the stairs and set it off near the cracked wall. This will open up a bonfire and McDuff's Workshop. Alternatively, you can use the Bastille Key to open the nearby door. The Bastille Key is in Belfry Luna after the Gargoyles.

The Dull Ember can be found in the large open room of Iron Keep. When you first enter, a sniper will be firing at you on your right. Make your way around and you can jump down to the platform under him, where the Dull Ember rests.