First things first, 6 forms in 5 minutes. Typically time limits wouldn't be a problem, except when the boss has a very good amount of health to outlast your attacks, as well as making sure you cease attacking when he attacks. In 9.0 especially, I had to get hasty with attacking so I don't time out, and if you DO time out...GAME OVER. You have to start all over to get back to the same position (why troll us so, Sakurai?).
Anyways, now to the boss itself...
1st Form: The Hands: Wow, Sakurai and pals, I didn't think you could make the Hands any harder, but you did it and then some. I'll say it out right now, the Hands are easily the hardest part of this fight. They are even more unpredictable than ever before, with little to no telegraphing involved, and their attacks can be used as a setup for an onslaught of pain, such as Master Hand's snap to one of Crazy Hand's ridiculously high knockback/damage moves. However, it is the cards (or cheese) that Master Hand deals that is the killer. Alone, it is simple enough to dodge, but then you have Crazy Hand, whose attacks can bury you on a card, one of those being a grab, pretty much guaranteeing a KO no matter what your percentage is. Essentially, you want to pray Crazy Hand doesn't get obnoxious when the cards are dealt (which is a slim chance, by the way). Some of the other attacks that will annoy people are the energy rods and fist punch from Crazy Hand. All that I could do is just make sure that I defeated the hands quickly.
2nd Form: Sapien: Not much of a first form for the Master Core, to be quite honest, but threatening nonetheless. It suffers from being like Tabuu, which means very deadly attacks that are easily telegraphed, but a few moves stand out. The stage lift attack is really risky, as it can easily rack up damage and trying to go off the ledge and what-not could result in a quick KO. His "Off-Wave" esque attack is really brutal in 9.0, easily KOing you if you don't dodge. But because of how easily his attacks are telegraphed, this is by far the easiest form.
3rd Form: Scorpion: From predictable and slow to somewhat predictable and rather fast, the Scorpion form can do some nasty things. The stage jumps are the real killer in this form, as there is no way to determine if the Core will jump on the same side as it was previously, or go to the other side. There is also the bite that it attempts by going off-stage, but that can be easily sidestepped or rolled out. The only projectile it has is a set of spikes which will jut out of the ground below you, but again, simply sidestepped or rolled out. The difficulty of this phase really just comes down to the frequency of his attacks, because it can either do a few at a time, or just spam every single move in its arsenal.
4th Form: Sabre: It should be noted that you can sneak a few hits off this form at the beginning of the phase, and it is heavily recommended. The range of this form's attacks are dumbfounding, but most have projectiles (so ready up reflectors if you have them). Only two attacks come to mind as threatening, the flurry of slashes to finisher and the 5-sword slashfest. The flurry of slashes is actually very simple to dodge, but it is very foreboding. JUST SHIELD every attack except for the finisher, in which case do a dodge. If not, the last hit will whack you downwards and far away. The 5-sword slashfest, as I call it, is easily the trickiest move in the entire Master Core arsenal. Swords 1 thru 3 go in a rhythm, but the 4th one comes in unexpectedly fast after, which kills the rhythm. Getting hit by the large 5th sword will likely equate to a KO, so yeah.
5th Form: Simalcrum: Or Clone Form, for some. Essentially it becomes a standard CPU fight with the catch that the opponent has much more killing power and potential than you, and starts out large, meaning it has high ranged attacks. All I can say here is to do your best, since it becomes a regular old CPU, and whittle down the form fast so that it becomes smaller. A standard KO against the form ends the phase, so try all you can to finish the fight fast.
6th Form: Smash: Have fun smacking away the core. Just don't wait too long, otherwise you will have to deal with Off-Wave esque attacks again before it self-destructs.