Try to bait and mix up your landings.
You have a LOT of options in this game:
-fastfall, or don't fastfall.
-double jump (can break your fastfall, can change horizontal aerial direction, and sends you up-- if you have an aggressive opponent that likes to do laggy uairs, this is a nice option.
-Of course, airdodge, but try to not land with it. Characters with a zair can retaliate in some situations. Mostly use this against uair-ers, since you fall past them, and don't give them another chance to follow up.
-If you have a special move that changes aerial momentum, you can use that as well. Know which moves have b-reversals or wavebounces, which ones break fastfall, which ones change horizontal momentum etc. Details about specials like these, and how they should be used should probably should be covered in the forum for the respective character.
-Go on the offensive and use dair/fair, or whatever. Use hitboxes with projectiles or aerials to create a wall between you and your opponent.
EDIT:
Forgot all about the projectile part.
I think you can still mix up fastfalls and double jumps and bait out projectiles, since really, all the projectiles in the game can't be spammed consecutively, unless you have a smart Link/Toon Link, and of course, Fox/Falco's lasers.
Fox and Falco's lasers are fairly punishable, and if they're too close to your landing spot, they probably won't be using projectiles anyway, since putting the gun away takes FOREVER in this game, and that animation isn't cancellable by any way that we know of right now. Anyway, when you have projectiles out of the equation, you can use the options I've stated earlier.
As for Links and Toons... bait out their charged arrow if they've charged it, and change up how you do that. At higher levels, Links and TLs generally don't charge their arrow, so don't count on that. Just watch for patterns, and think about how you can retaliate, given your options!