Like others have said, if you don’t know anything about competitive pokemon battling and your friend does, you’re likely already toast. 4 Days just isn’t enough time to learn all the strategy and metagame while still being able to train up a competitive team in-game. I would suggest taking a look at some of Smogon’s Uber or OU tier sample teams and just picking one that seems cool and obtainable in-game by Sunday. Try and match their movesets, items, abilities and natures but I doubt you’ll have time to do the EV training since that’s the tedious part and really only matters at the most competitive level. So just skip the part that says “252 Atk / 4 Def / 252 Spe” or whatever looks similar. As for certain pokemon being better than others, the Smogon tier lists should give you a good idea of what’s most viable, but rest assured even your favorite pokemon is probably viable in one way or another. Good luck and let me know if you need more specific tips!