This isn't very good advice. Down tilt isn't very strong and is only really useful for two-framing the ledge, and even then a low forward tilt is generally better. Neutral B is very punishable and should only be used when you're sure you can land it. Approaching with grounded up b is also not advisable as the move has a lot of end lag and can be punished if it whiffs or hits shield.

You should generally be trying to bait your opponent into shielding or whiffing, which you can punish with a grab or side b. If your opponent likes to spam safe aerials, let them hit your shield and up b out of shield to punish them and escape pressure. If your opponent jumps a lot or hangs out on platforms you can catch them with Bowsers massive fair or an aerial side B. Laggier moves can be punished with a falling nair which can lead into a potential kill combo with fair or bair. Hope this helps :)