The lightning gun excels at places where there isn't much room to maneuver. The following are places that I've found that are especially good for cornering people with the shaft:
 

• Anywhere on the RA platform when you're on even ground
  
• When enemy is coming from GL platform and you're at RA
  
• At GL platform
  
• At the 2 lightning packs corridor
  
• 100% Health Tunnel
  

Next: Surprising the Enemy