Either I'm bad or the Wii U AI is much harder. Probably both.
Even though I said it sucks, it is more that it is exploitable due to a linear play pattern. 9.0 can be tedious and tilt inducing if gone into without the proper mindset or strategy. I think the AI is relatively the same in both versions. I beat it with Mario, Lucina, Bowser, and Captain Falcon thus far. Generally I think defensive play and punishing the AI up to master core is the best strategy. You can use items and adapt to the stages better than the AI can.
The AI is better at shielding, grabbing, and dodging than it is at playing offensively since they have good reflexes, lack combo knowledge, approach knowledge, threat evaluation, and because they put themselves into vulnerable positions after using a move which has a long animation time. Then once you get to master core, you need to play aggressively since he has a lot more health than you do. You generally should fast fall and grab the ledge so you aren't as vulnerable after you use your recovery move if you get launched into the air. That is a common mistake which allows people to get juggled by real players, the AI, or master core.
I usually fast fall and abuse hitboxes. Bowser for example, can hit master core 3-4 times with his down B due to master core's hitbox being huge. It is also important because since master core is huge, your smash attacks can reach him if he is above you. Memorizing master core's ko moves are key, and memorizing what moves do damage. The reason for this is because some moves of his you need to dodge and some you need to shield. It is also important because you can play more aggressively when he is using a move to do damage to you instead of to KO you. He can be beaten without custom itemizations, but customs help a little bit. I play aggressively since you can get like 2-4 hits in between each time master core pauses.
You want to kill master core's first form as fast as you can, since he generally weakens you quite a bit. The scorpion form, you generally should play more defensive since he is easier to avoid and punish. Shielding and using smash attacks with any character usually works since he is on the ground. As for the swords, you generally want to move around a lot since you can dodge a lot of his attacks. His sword form can break shields too, so you want to kill him pretty fast.
Once you fight your clone, play defensive and treat him like any standard AI. The AI will probably misposition or waste a smash attack, and you can use those opportunities to punish him. I generally think it is easier to waste the clones health rather than trying to smash him out of the arena. I used speed heavy customizations, but I know other players have tried heavily defensive ones too.
Hope it helps! I am a mediocre smash player vs real opponents, but this information is what I used to defeat the AI on classic and all star mode on the hardest difficulty.