my brother plays a really good metaknight. Because he has no lag on his attacks, you're going to have to punish him extremely hard. Keep in mind that he is really easy to combo, so try to combo him with uairs, and uptilts. He's pretty much made of glass though, so an upsmash around 80 damage should kill him. Dont stay near the edge, its an easy way to lose against him.
Sonic is a litlle tricky, but keep in mind that he has really low priority spin attacks. Throw out neutral airs to counter the spin approachs, and try to lead into upsmashes.
DK is tricky, but try to bring him to platform stages like yoshis, or halberd so that you can't be comboed in the center of the stage. He's a heavy weight with slightly limited shield options, so try to pressure him hard, dont let him gain momentum.
Fox's worst matchups are mewtwo, falco, metaknight and a zelda/sheik combination.