Rock Lee is one of the easiest characters to use. All of his attacks and combos are really fast and his only justu just speeds him up and makes him more powerful. Lee is good because most of his attacks make him go behind his enemies. This one shouldn't be much of a challenge.
This is less of a method, more of a useful tip to help you get any of the Forest of Death Exam achievements. At the end of a battle, it gives you the option to leave feedback. On opponents you found easy to beat, such as people using Sasuke, Sakura or Haku (all of whom are normally easy to beat online), always leave a positive feedback. You will be more likely to face them in the future, making you have a definite win when you do (possibly).
This method will also help you to get exp towards the three rank achievements:
p.s. Please do not do the opposite of this and negatively vote those whom you cannot beat. It's unfair to them, and it will eventually lead to them having bad connection to the game. Only leave positive feedback on those that you can beat.)