Well with Pikachu the way I play him is I am usually always moving and jumping around and not letting up on my foe. Now if ROB is usually in the air I would suggest either chasing after him doing your Forward Air or Back Air, Forward Air is for a more direct approach as the Back Air is if the are trying to get around you so you can do the frisbee motion to fly around the specified area.
And when Rob is off the ledge there is no reason to be chasing after him since his Up B is like Pit's and allows him to go great distance, so the best thing to do would be to wait by the edge and Thunder or try to Thunder shock him while he is off because ROBs Up B has a limit unlike other people he doesn't regain his Up B after he gets hit he has what some would call a Gas Limit where he will run out of power and the only way to recharge is to land on solid ground.
As for the top you can either try thunder shock the moment he throws it to maybe stop it? I am not sure what has more priority though or you can try to time it right and catch it since it is still an item and can be caught, but the best thing most people would say is either Shield, Air Dodge, or Ground Dodge.
As for getting on the ledge without getting punished I think it is the matter of how you approach the stage because if I remember correctly if your going down on the stage Pikachu will sit there crouched for a moment but if you are slightly above the stage and fall onto the ground you shouldn't have as much lag time. Though another trick with Pikachu's Quick Attack is that on his first zip if you are hovering above ground you should be able to jump out of the Quick Attack animation and still use any moves but you just won't have any jumps until you completely land.
I hope that helps I probably could have helped more if I had my system to fight a ROB personally, but based on my game play with both ROB and Pikachu it should be ok.
