person1121

Story mode is the same as mission mode, you only need to beat one difficulty of mission mode.

For the Kakashi vs. Itachi battle, I'm pretty sure you've already seen that attacking doesn't do much good. In general, try to block most of his attacks, stay away from using guard (back A), and use attacks that knock him down. A couple suggestions are:
1. Use Kakashi's underground attack (down A), make sure he isn't dashing or moving away from you when you do this.
2. Use Sharingan, although this should be a last resort, as it does not add too many knockdown attacks to your available ones.
3. Use fast attacks, particularly Kakashi's back weak attack (the three hit combo), because back attacks block hits for the first 1/2 second or so when you use them.