Level 9s in Brawl are not too difficult, just remember a few things:
1. Watch the shield grab...god I hate the shield grab
2. You MUST abuse your characters strength and their characters weakness. (I play as Marth, so in most all cases I take advantage of the little extra range Marth has to limit the cpu's ability to hit me while I can still hit it)
3. The difficulty ranges depending on the character. Space Animals, Luigi, Meta Knight, and Link are some of the better AI for some reason. Bowser is pretty awful, on the other hand.
4. As a CPU, they can do things not humanly possible, but also do not use human reason.
The CPU can press buttons at the speed of light. This can be bad for you, but I wouldn't worry too much about it. An example would be Meta Knights Side-Tilt attack, which is a three hit combo. The CPU can execute all three attacks faster than any person could. This can give them a slight vulnerability advantage if they miss, but nothing serious. On the other hand, I've fought an Olimar CPU on more than one occasion who uses 0 or 1 Pikmin. Why? I don't know, but I never realized how dangerous Olimar could be without any Pikmin.
5. They don't make mistakes...well, not many at all. Just cross your fingers for a trip every now and then so you can really make them pay
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6. Oh, and if you plan on doing a lot of lvl 9 fighting? Stay on your toes and prepare to have tired fingers after a while, because they'll keep you busy...
I'd love to hear when you finally get consistent wins on level 9 opponents. It's a landmark step in any Smashers career!
Happy Brawling!
~BadAxel97