To earn the Talonted achievement/trophy in Gotham Knights, you must “Defeat 40 Feral Talons, 25 Gladiator Talons, and 15 Hunter Talons”.
Feral Talons, Gladiator Talons and Hunter Talons are all parts of the Court of Owls faction, and you’ll start running across them during Case 3.2 - Chelsea Tunnel… Feral Talons, at least, the other two enemies won’t spawn until later. Gladiator Talons will appear in Case 5.2 - Inside Gotham’s Walls while Hunter Talons won’t appear until Case 6.2 - The Voice of the Court.
To hunt down these Court of Owl Talons, just grind out crimes (perhaps in pursuit of legendary gear or while trying to thwart 250 crimes for Expert Crime Fighter?), but take the added steps. Look at all the premeditated crimes on the map (you should be screening them for legendary drops anyways) and if the faction listed is “Court of Owls”, clear it, as you may get some Talon kills this way. As for opportunistic crimes, hover over each district of Gotham City on the map and look for the owl icon under the “Faction Presence” header - if you see it, head to that district and start hunting the white triangles on the map (and compass) marking opportunistic crimes, as at least one of them will spawn Court of Owls enemies. Repeat until this achievement pops.
Keep in mind that you can continue to hunt Talons in the postgame, but if you start New Game+ your story progress will be reset. To get Talons to spawn again on New Game+ you’ll need to progress the main questline to Case 3.2 (Feral Talons), Case 5.2 (Gladiator Talons) and Case 6.2 (Hunter Talons), respectively. That said, two playthroughs of the story might not be the worse thing in the world, especially if you need the XP to reach level 40 for the [He’d Be So Proud Of You] achievement.