A tougher version of Kang Roo, Shadow Boxer K attacks nonstop at a faster pace than the normal Roo and has more health, attack power and knockbacks. She also has area attack, something Kang Roo doesn't have, giving her the devastating power to plow through your defense if the damage isn't enough to knock her back. This black brute is notably slower than most other Black enemies, at a speed of 22, slightly faster than the original Roo, compared to the likes of something like Doge Dark and Gory Black, who share a movement speed of 30. This is considerably the strongest Black peon in the game and shouldn't be underestimated, especially when spammed.
As with Gory Black, a good way to deal with this enemy is constantly stacking mid-ranged Area Attackers, such as Figure Skating Cats, Salon Cat or Weightlifter Cat. Any Area Attacker with a range over 250 should be fine. You can use Bahamut Cat if you like, but Shadow Boxer K's tendency to get knocked back (which causes him to miss often) and the fact that Bahamut Cat gets overwhelmed easily is why he should only be used as a "big red button" against numerous Shadow Boxer Ks when their push power exceed your defense capability. Awakened Bahamut murders Shadow Boxer K like there's nothing to it, but be careful to protect him with meatshields because he only has 200 range, which is fairly low even compared to Shadow Boxer K's range of 158.