My Ganondorf is to up 15 KOs in Cruel Smash. Here's my notes for Ganondorf and the mode in general:
1) Note where they Miis spawn. If you restart, they will ALWAYS come in in those spots at first. Determine if they all bunch up or there are safe spots.
2) Either aim for them all with an UAir, or get to safety on the other side.
3) Miis in this mode aren't very aggressive at all. Your most common hits will come from Brawlers approaching you from the air, or the last hit of a Gunner's FSmash (Note: if the main part is missing you barely, shield as it ends. The last hit has a longer hitbox).
4) If any Miis come within range, DSmash to pull them in and send them off. When recovering, the only 'offensive' move they'll ever use are footstools (and unlike Brawl, you do NOT want to be footstooled). If a Mii recovers high (the only one that can get back on stage), jump and UAir him to confirm the kill.
5) Unlike Brawl, your ledge attack CANNOT send any Miis offstage. The 'Alloy Exploit' is gone, but a ledge attack can still deal a little damage and make some of them roll towards you so you can DSmash them for more KOs.
6) If a Mii spawns above you, note the type quickly and choose the appropriate response:
Brawler/Gunner: Uses DAir to hit you as you stand. Jump and UAir preemptively, or spot dodge and DSmash.
Swordsman: Uses DAir. Due to the landing having a hitbox, roll and find some way out of the area, then hit with something, even just a jab.
7) Avoid clanking/clashing with attacks. Thanks to their ridiculous damage potential, you'll be stuck in the animation for what feels like ages, unable to do anything.