I have some experience in three areas of WiFi battles;
Battle Spot - Rating Battle
This is where the lion's share of the competitive teams are. You may get lucky at first, as the system seems to attempt to match you against someone near your rating. And if your team (casual or not) has a decent move-set, type distribution, and are EV trained - you should stand a fairly decent chance.
Advance your rating too far, however, and you will start to get into the more competitive teams. At this point you may be cursing your earlier good luck at being paired against teams you could beat, as the competitive teams start moping the floor with your pokemon.
Battle Spot - Free Battle
Free battles are more of a mixed bag. On the one hand, most trainers attempting to show off their skills and earn bragging rights will be in the rated battles. On the other, free battles aren't as restrictive and allow all sorts of legendaries and load-outs that may otherwise be restricted. So while sometimes you might go against some trainer with 6 Magikarp, other times you're up against all three Kalos legendaries, a Mega Mewtwo, and other dirty tricks. Granted, having all those legendaries doesn't by any stretch of the imagination make them invulnerable (personally, I find that most trainers who overload their team with legendaries seem to overlook the fact that they still need good move-sets and strategy), they often do have higher stats and hard-to-deal-with moves.
Like I said, a mixed bag. Enjoy the battles for what they are and have a thick skin. Sometimes you'll wipe the floor with them, sometimes they'll hammer you into the ground. But other times you'll get just the right mix and have a suspenceful battle that could go either way.
For what it's worth, my field team, whose held items are largely comprised of things like Amulet coins, and includes things like a male Combee, A HM slave Diggersby, and a Smeargle whose only offensive attack is False Swipe - This field team has a ~50% win rate in free battles (if you include the opponent's rage disconnect as a win - I know I do).
Player Search System (PSS) Battle Challenges
Challenge a random passerby to a battle. This is another mixed bag of a different sort. While you could roughly tell the expected difference between free and rating battles, You don't really know what to expect from passersby. If they accept your challenge, do they have a competitive team in the wings? Do they have a terrible team, but are bored? Somewhere in the middle?
Typically though, those who have challenged me from their passersby list, with the exception of one, have all been curb-stomped by my semi-competitive team.
I have only had one person battle me repeatedly enough that I added him to my 'friend list' (I later removed his friend code from my 3DS) and battled him again with the game chat enabled. So I can't really comment on if there's a lot of 'smack talk' or anything that goes on. My one and only voice-chat enabled battle was with a French kid who didn't say a whole lot.
Best Answer
If you don't know anyone you can always battle random people in your Passersby list. Or you can easily find challengers on forums or whatever, add their Friend Code and become friends on Pokemon.
You can find these features in your Player Search System (PSS) page. In ORAS the PSS is accessible via the PlayNav app of the PokéNav Plus.
Everything you need to know about how to challenge people or send trade requests is explained here.