@Professor-P: To prevent getting uair combo'd x3, the basic advice is to DI the up throw behind the Fox and spam forward air. If they have good timing, the forward air won't really come out or hit, and you should DI that first uair in the opposite direction that you were just DIing. So basically go back and forth in the air. It forces the Fox to jump in one direction, putting them as far away from you as possible.
As for tricksies...
If you see him run at you, just stand there and grab. You are Marth, king of the grab game. If he starts jumping in instead, dash back, turn around and grab. If they laser spam you, get close, then wait for one of the first two options. If you really need to start being offensive, get close and force a spot dodge, then grab. If they start getting used to you anticipating spot dodge, just immediately grab.
Starting to see a pattern?
@Witchking: "Wavedashing, dash dancing, and Fox trotting are your movement friends."
These are the basic movement tricks. Once you start learning how to abuse wavedash momentum, you can really start mind****ing people.