The Katana only deals 80 damage, but combined with the 40 damage and long range of its dash attack, it can get kills from a range much further than any other melee weapon.
It is very much a combo weapon.

Enemies hit by a dash attack will be knocked over, but quickly securing a kill to get the cooldown reset is important for survivability.

• You can dash immediately after an attack, dealing lethal damage instantly.
  
• You can also attack mid-dash with the proper timing.
  
• Without a dash up-close, you can attack + kick to instakill an enemy.
  
• Getting a kill with any weapon will reset the dash cooldown, so switching to a primary after dashing is viable.
  
• It's best to save your dash until you can confirm a kill.

Playing with Lordoloa's Executions Mutator will also give you valuable i-frames during executions, which can save you in a pinch.

Good luck!