MK: It takes patience, spacing, and knowing exactly what moves beat what. Grab a lot. Dthrow is one of your best friends in this MU, as is pummeling to refresh that undoubtedly stale Bair when he's at kill percents. At low percents, Dthrow>Utilt>Shieldgrab>repeat is easy to land until about 40%. The only thing they can do is get a footstool jump (requres strict-ish timing) or buffer an AD (But if you predict it, you can punish it very hard with Dsmash/Usmash/Fsmash/Hammer).
Generally being under MK is, despite what you might think, rather good, so long as you know his options and react accordingly. Pressure his landings and punish them.
Offstage, you have to be careful, but almost every aerial beats SL so long as you're above him and/or behind him. If his sword can hit you, then he will beat whatever you have out.
If you need help with more specific things in the MU, ask, since I'm not really sure what else to say. (Oh, and ban Halberd or Brinstar vs MK IMO)
Diddy: Not quite as familiar with this one. Nair before you land is a good way to grab bananas. Air camp him a lot. JCIT is pretty helpful... Gimp him every time he tries to recover.