I play defensive projectiles against Marth, Sheik, and Peach. It's better to get a better matchup in my opinion agansit Marth because he totaly destroys Young Link's everything. Bombs are best used against him due to the fact if he trys to hit them away he gets blown up in the process, unlike the Boomerang and Arrows. My tatic against his is basically keep your boomberang out there towards him. Like I run away and stay planted on the ground while throwing the boomerang back towards him in the upward direction because every Marth usually jumps over it if thrown straight. You never want to use the boomerang in mid air because it will lead into a giant gap for the Marth to sneak in. If they catch on to that then they'll try to stay planted and watch. When that happens throw the boomerang at them. While this is going on keep bombs handy. Easy thing to do against Marth is bomb throw at him into a Nair. Easy damage and it's 90% safe as long as the bomb hits. Same thing can be done against Sheik except when she become needle spammy. Then you'll have to rely on bombs coming from above into Nairs or Dairs.
And against most others I go aggressive pressure. Usually I SHFFL all my ariels.Fast Fallers like Falco, Fox, Falcon, Ganon you want to always try to stay on the ground. If the Falco goes for pillars then you want to intercept with Uairs then just keep up with the Uairs because he has a hard time recovering from a hit in mid air. With all the characters in mind you want to try a Utilt because it can set them up for many combo kills. Usually Utilt/Uair can start these and they are continued with the SHFFL'ed ariels.
Gah I hate it when I type too much. I should just try to attempt at a guide for all this craps. -.-