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As a King Dedede Main Projectile users are the bane of my existence, especially when it comes to For Glory Standards. Not only is King Dedede a huge, hittable target they can easily shut down his own projectile (Gordo) with their own so easily rendering it just about useless. Since they also don't necessarily have to approach due to their projectile game it makes his Bait and Punish harder to do as well. He doesn't have too much going for him as one wrong move/read could have deadly results to begin with.
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I have quite a few friends who play Ganondorf, and given the fact he can be combo fodder he's also pretty doomed once Bayonetta gets him off stage. A good Edge Guard read could result in Witch Twists or whatever else you can think of, and one thing I love to do is Witch Timing Ganondorfs Up B (timing it so where the little slap at the end activates it). Afterwards he's at your mercy off stage during that. Little Mac is pretty much the same story. Bayonetta can Edge Guard him to death and he can't do much about it.
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Small, tiny, and quick does not make a fun time for King Dedede. Complete combo fodder for Pikachu and due to how small he is he can be pretty difficult to hit. Luckily I hardly ever find Pikachus, but when I do I know I'm straight up dead. RIP all hopes.
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When I used to play Pac-Man (not like I was a very good one anyways) Rosalina always meant destruction for him. Her Gravitational Pull could shut down his projectile options as well as trap options. Plus Luma likes to be a meat bag too.
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Okay, maybe not the worst but one thing to take note of is Villager can easily take away one of ROBs greatest tools (his Gyro) and keep it from him for just about the entire game and even use it against him, giving Villager an extra projectile option. Plus ROBs size can make it slightly harder to avoid his Slingshot, and you better be careful when you think about reflecting Villagers stuff back at him unless you want him to pocket those, too. It's not terrible but it's really annoying in my experience more or less.