Deitatus
Stalkin' yo profile page
For the main storyline, the best thing to do is probably go online, look up the Pokemon you'll be fighting and train a team that's strong against those Pokemon. But in general, yes, the best thing to do is make sure each Pokemon's weaknesses are covered by another Pokemon on your team (ex. Snivy's weak to Fire, Panpour covers Fire. etc). What you have is a bit rudimentary but it works for in-game. Things get way more complicated in competitive battling, though.
Ikr. I just took the game guide's info. I never used it, but it works.
Simply put, the E4's types are Ghost, Dark, Psychic and Fighting. For the Ghost and Psychic, simply get a dark type pokemon. I recommend Bisharp. Despite Bisharp's failure as fainting a lot, its a great pokemon to have. Teach Night slash, and that will work against the Ghost and Psychic pokemon easily. Then teach it some fighting move. That'll help against teh Dark Pokemon. For the fighting type, I'd recommend a flying type. That will also help against Scrafty a Dark/Fighting pokemon.
I'm not sure whether you fighting the E4 first time or the time where you fight the champion. That would help >.>
@Grimdour
Lol, nice. I never thought of using a Chandelure. I swept them with a Bisharp, with a quick claw attached to help against Marshal.