Gaara is one of the easier characters to get through the Forest of Death exam with, IF you know how to use him. He has several powerful combos that can prevent the enemy from attacking very often. To get the 5 win streak there are a couple of techniques your going to need to master:
Substitution: Easy to do in the story, even easier online. This is when you block exactly as an attack hits you, and you teleport behind the enemy. This technique is endlessly helpful against anyone, as it gives you an opportunity to pull of a powerful combo, possibly leaving the enemy stunned, which would then give you time to use 'Sand Burial', which is the other technique you will need to master.
Sand Burial: This jistu, like Kikashi's Lighting Blade, is very useful, as it's easy to pull of and hard to block from by your opponent. I would recommend using Sand Burial purely as a finishing move rather than a regular attack.
Rage: Rage mode is very useful, but only if used right. Almost everyone online knows how to stay in the air by kicking off of your head while in rage. The biggest problem with rage is when your opponent uses it during or after yours. My advice is to only use rage if the opponent has already activated there's. To use rage, when you are on very low health, or have been attacked a lot, press both triggers at the same time.
And a useful tip: 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.)