Return of the Tips
#10 --- Beware the Kyogre that knows Calm Mind, which raises the Pokemon's Special stats. Combine this with the fact that Kyogre's special ability, Drizzle, makes it rain constantly and the fact that Kyogre can learn Thunder, and you've got yourself someone deadly. Either send out Rayquaza to neutralize the Drizzle ability, use physical attacks such as Return or Earthquake, or both. Another suggestion is to use Sunny Day immediately after Kyogre arrives.
#9 --- Know your special abilities; most are good, and some are bad. For instance, a Pokemon that knows Protect or Dig can overcome Slaking's Truant ability, which makes it move every other turn. Other abilities, such as Glalie's Inner Focus, can help frustrate an enemy.
#8 --- Master the 2v2 matches. Try using Pokemon that can compliment the other's weakness(es) (ex. Wailord and Ninetales); an attack-reducing strategy (ex. Masquerain and Mightyena; works only when first sent out); or even an attack-raiser strategy (equip a physical-move user with a Persim or Lum Berry, have your other Pokemon use Swagger against him, and there you go). The possibilities are endless.
#7 --- In 2v2 matches, be careful if you're using moves such as Eruption or... *shudder* ...Explosion. You'd probably feel a lot safer if your other Pokemon knows Protect or something.
#6 --- Shedinja may seem tough with his Wonder Guard ability, but remember he only has 1 HP. In that case, always keep a super-effective move with you (Rock Tomb or Flamethrower, hint hint). If you can't do that, try to build up resistance to his moves and hope he runs out of PP before you do; try using a faster, Recover-using Pokemon. When all else fails, use Spite or Toxic.
#5 --- At the Contest Halls, remember that winning isn't just in having the best heart-gaining moves. Heart-jammers and combo moves can work wonders, too; get the hang of those, but don't rely solely on those either. Try to anticipate what the other guys are going to do.
#4 --- The Battle Tower may be slightly easier than the one in Pokemon Crystal, but it still gives me fits; the battles get harder as you go. Thankfully, the prizes are worth it. Anyway, I can't tell you exactly what guys to use, but make certain that all their weaknesses are covered. Oh yes, and a word of warning: once you've gotten at least 35-42 victories, the matches become a whole lot harder (especially since you'll be dealing with the Reggies, Latias, and Latios from time to time).
#3 --- Get a lot of friends. Linking up, mixing records, and fighting the trainers within the Secret Bases will do your Exp. points a lot of good.
#2 --- Save the TMs for the male Pokemon. Male Pokemon are the only ones that can transfer moves to an egg Pokemon via breeding (unless they both know the same move, but that's a different story).
#1 --- I can't stress this enough: to you beginners out there, the Hoenn Elite Four (plus Steven Stone) will chew you up and spit you out like a wad of used chewing gum, so don't only on whatever TMs you find. Raise a varied team of at least Lev. 50, and have a wide variety of move types---everything from fighting to electric. A word of warning: Steven's Metagross + high attack + Meteor Mash + possibility of raising attack = **PAIN!!!*** Stock up on Hyper Potions and Revives beforehand, or you'll regret it.