You get this achievement by killing a 'Proddy' enemy as Bolt with a finishing move.
Proddy are the female enemies with the Tonfa, two baton like weapons.
These enemies start to appear after the first few missions.
A finishing move can be used when 'B' appears above the enemy's head - Hit 'B' and then hit 'Y'.
The 'B' will appear above the enemy's head after a certain number of Bolt's normal X/Y attacks.