Assassin's Creed Rogue

The enemy that uses smoke bombs, not grenades, the most are Assassin scouts (or agiles in AC IV). If you have problems identifying them they are dressed in orange, are the smallest of the enemies you will encounter, and the only decent way to defeat them in combat are by countering their strikes as normal attacks they will consistently block. Now that you know who they are, the best place to find them is in New York.

Regardless of how many hideouts you have liberated in New York, Assassin scouts are pretty easy to find. The best place I found was near the docks in Lower Manhattan. Using the fast travel point in the southwestern portion of that area, you go east towards the ocean, and then head south down the street. This is also a great place to grind out Denied and/or Stalker Killer. You should be ambushed by three assassins, one from up high, and a scout will almost always appear (if they aren't one of the attackers). If this approach doesn't work, finding a scout is very easy near forts or up in the northern parts of New York.

Once you have a scout, when you counter their attacks throw them with (instead of attacking them with ). Keep your distance and around 75% of the time they will throw a smoke bomb in your direction. Use to counter the smoke when prompted (check upper right controls), and then counter and throw again. Rinse and repeat as necessary. If you somehow kill, or the scout is killed by someone else, run around and find another one or fast travel to a good location as all enemies will reset.

A word of caution: I'm not sure if the Uplay exclusive outfit works with this trophy as it has a gas mask on permanently. I'd use any other outfit but this one to be safe.