Metaknight has 4 approach options, but really only three.
1. Mach tornado:
Pk fire it. it suddenly isn't an option any more, and now he has 3 options.
Now I'll start the real list.
1. Dash in and grab:
When I play metaknight, this is my favorite approach. If you see this coming, Jump up and back. This leaves you in a place where you can punish this and his dash attack (#3), and won't get hit by his short hopped aerials.
2. Dash close, then SH Fair:
Guess what? Meta moves slowly in the air. Back off, then punish with either a projectile or a jab combo, stick, really even a grab will do.
3. Dash attack:
Full hop Dair takes care of this nicely. Or treat it like a dash grab, because that'll protect you from his Short hopped Fair better.
Never approach the metaknight. He has to come to you because of the fire, and when he does, you can use the fact that he only has a few solid options to your advantage.
If he grabs you, expect a downthrow, or a bthrow off the edge. He'll try to either grab you again, or SH a Nair at you if you pop into the air. DI away, try to jump out of it, the air is relatively safe due to his slow aerial speed.
Don't go into the air too much, though, or he'll dash to where you're about to land and do something nasty, like shield then Up B (which kills pretty well)