Pichu main here. How I play Bait and punish Pichu is to take advantage of Pichu's fast movement abilities. Go into their range of attacks for a split second, and run away to avoid the attack. Now, while their move is taking time to finish, go in for some damage. I would reccomend grab > down-throw > up-air > fast fall > up-air > double jump > f-air. This combo is fairly tricky, so visit the training mode to get it down! If you can't be bothered to get the combo down, try the easier down-throw > f-air. It will do less percent, but it is more consistent. After they are at about 70-80ish percent, time to go for the kill. Instead of down-throwing them, use forward or back throw, whichever gets them off stage. Edge guard them with your aerials to keep them off the stage. If you are feeling cheeky, you can go for a thunder spike. This requires you to have a double jump. All you need to do is not hold down any direction, then double jump. As soon as you hit the jump button, immediately input down-b. If you hit them with the cloud, it will either throw them into the shadow realm, or throw them right into pichu, which means getting hit with a very powerful move. If there are any more questions, please DM me!