You prefer to deal sublime punishment to a single foe at a time - not in a duel, mind, because a duel implies chivalry, and you're too busy getting the job done for honour.
Assassin is a Subclass of Rogue. It's focused on dealing higher and more reliable damage to enemies at the start of battle, especially if you can surprise them.
Subclass features
Level 3
- Quick as an alley cat in a rain-dark city, you immediately restore your
Action and 
Bonus action at the start of combat
- Any successful Attack Roll against a
creature is a Critical Hit.
- You are deadliest against unprepared enemies. In combat, you have
Advantage on Attack Rolls against creatures that haven't taken a turn yet.
Level 9
- Adopt a new identity, changing your appearance. You can go
for 10 turns.