I just did it with Ike. I used Legendary Sword, King's Cape, and Auto-Healer Cape (+85, +75, -101) for equipment and Furious Eruption, Unyielding Blade, Aether Drive, and Smash Counter (3323) for special moves. Furious Eruption can be charged between forms of Master Core and can hit the higher cores in Master Fortress pretty hard to avoid dealing with enemies that spawn after hitting the cores. Unyielding Blade gives super armor to approach/kill CPUs. Aether Drive gives horizontal and vertical recovery. Smash Counter destroys crowds of CPUs, the Hands, Master Beast, and Master Blades. Avoid BS stages and countering CPUs if you can, and don't yield your blade unless you need to recover or counter. Against Master Cores forms, use aerials or U-smashes when you can't counter. I countered Master Beast's electric attack, and the counter lasted long enough to avoid the whole electric attack.