It's hard to do the Assassinate the Onryo Samurai quest because of a lot of soldiers around him but there is a easy way.

Guide

1. Jump right in front of him to get all the soldiers and himself to follow you.

2. Go up one of big buildings and wait for them to lose you.

3. While they are looking for you, find Onryo and assassinate him with jump from a wall or anything close to him.