These days people are seemingly starting to get the message. It's been rather rough sometimes, and I get my N-airs punished all the time. I really don't abuse it that much these days. If I do it, it's from shorthops and more sparingly or just defensively. From the air, I try to only use it as I fast fall past them, or perfectly spaced from the front. It's just too easy to shieldgrab. Any aerial approach in the game suffers this, unfortunately... It's extremely tedious to fight Greninjas or Dr. Marios and others when they shoot their projectiles all day, because a good player will shieldgrab any attempt to go above their stream of projectiles and, there's really not much you can do because countering any aerial in the game is as simple as standing and blocking.
I've been laying back on air-to-ground, unless again, it's crossing up or I can space it perfectly. I just jump around them a lot with empty landings or just coptering around with N-airs for no reason, just not trying to attack them. Close the distance and try to make them lower their defense, then an occasional N-air works wonders. Alternatively, empty landings into grab can really throw people off guard, since Jump fastfalls so fast.
Speed has been more prominent for me lately, as well. Similar tactics, with occasional spaced N-airs, and making use of the amazing pivot game. Close the distance but stay patient, and use pivot tilts and grabs to apply pressure and punish. Just make them nervous by dancing around them and feinting a lot. Pivot grabbing is one of Shulk's most fearsome offensive abilities, it happens in a flash and is very hard to punish if you slide past them. Opponents tend to get more aggressive when you're bullying them with Speed, and sometimes will take into the air, where you can make use of those amazing aerials.
And something I've been doing quite a lot now, is short-hopping towards them with an immediate F-air, and then spacing back mid-attack. It has some really nice range and is pretty safe since you backpedal away while striking. Can be a bit hard to pull off the shorthop and F-air at the same time, though. In Speed, it works pretty good with full jumps. In Jump, you can, instead of backing away, just swipe and then keep flying over their head. This is more or less effective depending on character height, but I find it's been a safe offensive method. I think it's actually extremely useful, it's been rivaling N-air for me.