hmmmmmm well ive only been to one regional, the rest are all here at school wth crap players. BUT
my advice wld be when you go in, remember you are pika. u have crap range...takle a few minutes to play it super safe to see how your opponet plays and then chose your MO from there. like if they roll alot just watch and see how u can punish it.
i find it useful to run in at the start and just shield. see what they do. do they spot dodge? if so they are anxious and your dsmash will work well. do they roll away? if ur dealing wth a roller just watch how and when they roll, do they roll to get behind you or away? if they do nothing or they smash, they are either agro or passive. just plauy smart really haha
i didnt make it thru my pool at that regional cuz i was brand new haha. mostly try to do your best and learn from it. make sure to get in friendlies as much as you can, they are super helpful.
and lastly, remember its a game, HAVE FUN!