This achievement unlocks automatically through story progression and is not missable. It is unlocked upon completion of the final French Ward story mission,
Kill "Uncle" Lou Marcano.
Kill the Judge is the French Ward story mission prerequisite which, in turn, requires securing the French Ward Sex and Drugs rackets.
Completing
Kill the Judge unlocks
Kill "Uncle" Lou Marcano and directs Lincoln to talk to Donovan about Uncle Lou. When Lincoln gets in Donovan's Van, a series of cut scenes provides the necessary background. Donovan drops Lincoln off near a coal dumper that is poorly guarded by three inattentive enemies. Lincoln must connect four primer cords and then flip the detonator switch. Each of the four primer cords and the detonator switch are identified by objective markers.
As shown in a cut scene, setting off the explosives causes the coal dumper to collapse onto Lou Marcano's river boat as it passes by. Lincoln quickly swims to the boat avoiding a couple of alligators. On board, he is tasked to get to Uncle Lou. He must fight his way from the first deck to the second and third decks following the objective through the third deck dining room. Following the objective marker, he climbs either one of a pair of ladders on opposite sides of a platform. An
prompt appears on a hatch in the center rear of the platform. Pressing
triggers a cut scene where Lincoln confronts Lou on the boat's stern.
The confrontation is cut short as the boat's wheel breaks free, throwing Lou and Lincoln into the swamp. Regardless of Lincoln's previous health and curatives, he finds himself with no adrenaline shots and only one health bar. However, although Lou shoots at him, all shots will miss so Lincoln's diminished health is not an issue. Lincoln simply pursues Lou who shoots wildly and completely ineffectively. When Lincoln catches up to Lou, an
prompt appears. Pressing
results in, as Lincoln puts it, sending a message to his brother Sal.
After some cut scenes, Lincoln walks in to a sit-down session. The achievement unlocks as soon as he does so as shown in the following video: