Here's what I think:
Olimar: Like was said above, Oli is extremely predicable close range. However, I prefer to play a long range game and keep him away and rack up damage with Din. Avoid the pikmin as much as possible. When he's around 60% I like to move in and do a dtilt into dsmash then edgehog. Theoretically that's the best way to go about it IMO, but I don't know how your olimar players play. I'd play olimar in a platform stage, like Battlefield or Smashville. His pikmin throw gets disrupted and you have room to avoid his combos.
Samus: Simple. Build up damage with Din's (same as above). Avoid falling for Samus's missiles and plasma combos. Try your best to avoid them (airdodge or run away). I would say use nayru's love on them, but an experienced Samus player will follow a homing missile with a plasma ball (charged of course), so unless you have impeccable timing, or the Samus is a scrub, I'd just spot-dodge or air dodge them as best you can. Samus is a little tricky to kill on the edge, especially an experienced one. Do your best to keep them away from you and rack up damage. Don't go for a kill until you know it will connect. You should have the upper hand when it comes to spacing your smashes and aerials (except for samus's uair). Luigi's mansion is a great choice for me against samus. Samus can't do much if the mansion is up and you have the upper hand range wise with din's.
MK: Extremely tough fight for anyone. To counter his tornado, a well-charged din's will take him out of it. And if you're feeling gutsy and somehow have a good lead, your Dair will also go through the tornado if you get him from above. Your Fsmash should be able to go through his drill if you space it right. MK is also really prone to a dtilt into a dsmash. if MK trips during the dtilt, an fsmash will take care of that too. You gotta play extremely defensive with MK. Don't get greedy. That's the most important part. I recommend Luigi's mansion here too. Your survivability goes up, and you can refresh your usmash and fsmash with the pillars on the bottom floor. Just be sure to not get caught in a tornado while on the bottom floor and you should do fine.
Diddy: Play with his bananas. If you don't know how to glide toss yet...LEARN IT NOW!!!! Zelda has one of the best glide tosses in the game. If you can pull it off then do a usmash or fsmash, it's game. Really mess with diddy's head and reflect his bananas. It'll catch him offguard, and if you time it right you can follow with a dtilt combo. Edgegame wise, just spam din's. Avoid an edgefight with diddy. Other than that, you shouldn't have too much problem if you can space right. Most of your attacks outrange his. Corneria is a great choice. The crooked ground messes with Diddy's glide tosses, and you also have the bottom part of the stage to do an infinite dtilt against the wall. Plus, the low ceiling allows for easy kills for zelda. Just remember, diddy has the exact same advantage with the low ceiling so don't get hit.
Hope this helps. GL at the tournament.