The Wall Breaker is the fifth Elixir Troop unlocked in the Barracks once it has been upgraded to level 5. As its name suggests, Wall Breakers favor going after Walls, dealing massive damage to them.
The Wall Breaker attacks by locating the nearest protected (read: Wall enclosed) building and destroying its protective Walls, blowing itself up in the process. It deals splash damage around its target and deals 40x damage to Walls.
Wall Breakers prioritize Walls above all other targets, and will completely ignore any building or enemy troop while even a single segment of Wall exists on the battlefield.