I'd suggest playing who you're comfortable with honestly, but Dedede would probably be the easiest. Gordos can get three hits each for massive damage per sideB, and the Armored hammer downB lets you tank attacks rather than dodging them. Ganondorf is also easy for similar reasons (big damage from usmash spam, can armor with Warlock Punch).
Edit: Meant to add some stuff last night, but my internet cut out
For equipment,
I stack attack badges, with one being Critical Hitter. Even if your equipment is bad and you can only get ~90 attack or so, critical hits are ridiculously strong and potentially let you kill Master Core forms before they even attack with hard-hitters like Ganondorf. You get a guaranteed Critical Hitter Brawn Badge from getting 300 total KOs in Smash mode.
Phase specific tips:
Against the Hands, jump at Crazy Hand with aerials at the start. If Master Hand slaps, he'll fly under you and they'll both hopefully be dead before they do anything else. Airdodge Master Hand's attack otherwise.
Giant's pretty slow to attack, so reacting shouldn't be the hard part. If you're unsure of how to dodge some of the bigger attacks, jumping offstage puts you out of the range of just about everything.
Beast is "2 attacks at the start and after the facesmash, 1 after anything else. Jump and airdodge away immediately after." You don't need to know what he's actually doing, "jump and airdodge away" works against everything he does.
Considering Sword faces left, it has a hard time attacking to the right, so unless you push it to the right ledge you can run past it to avoid all of its "normal" attacks. When the swords spread out, the middle sword's glow spikes you if you hit it; if you fail to dodge the first four, you can airdodge after bouncing off the ground and avoid the fifth killing blow. Again, offstage is also a pretty safe place for this phase.
The clone phase copies your equipment; since attack equipment lowers your defense, it'll be pretty glass (though the same is true for you). The easiest way to kill it is by trapping its initial landing with a projectile or other long range attack (Corrin fsmash, Warlock Blade, etc). You
can hit it as it spawns, but the timing's tough and it's probably not worth trying unless you know it'll work.
Saw you main Corrin, so
I recorded a run with her. Couldn't record audio, so...enjoy RPG boss music I guess (it might be loud, sorry). Kinda sloppy (countering Giant's headsmash was a bad idea, I forgot when I could get two hits in vs Beast, and Clone gave me an easy fight despite screwing up the start), but hopefully gives a better idea of what I'm talking about/what to do.