When you see "Attack!" in the box, click Parry immediately.
< 120ms: Godlike
< 140ms: Elite
< 160ms: Pro
< 180ms: Semi-Pro
< 200ms: Advanced
< 220ms: Good
< 240ms: Average
< 260ms: Below Average
< 280ms: Slow
≥ 280ms: Noob
Using a fast computer and low latency / high framerate monitor will improve your score.