Huge wall of text will ensue. This guide will not spoonfeed you answers. I lie, it will.
GSC is infamous as a metagame that a lot of older players call "stall heavy" and easy to make a team for. You may have heard some of these players talk about GSC to something of this effect:
"lol gsc is just stall
ez meta
my own very original team was the best
raikou/miltank/snorlax/skarmory/cloyster/suicune
lol 198-0
i has big peen"
However, GSC is much deeper than many players will let on or even know. Admittedly, I myself didn't fully understand GSC until much later. Point is, I would never, EVER take my own advice from the previous me. Odds are, these "elders" you refer to were probably people like me, who thought they knew everything there is to know and house a very respectable win ratio against even the best players, but in reality have very limited knowledge of the game.
A quick word about team building: jack of all trades, master of none. Over time, you'll find that a "balanced" team, so to speak, would ultimately be less effective than a team dedicated to its own synergies. This is based on my philosophy that by controlling tempo you have the best chance of coming out with a win. If you plan on playing a fast-paced game, you shouldn't add stuff like a spinner or a cleric. Likewise, playing a slower-paced game, you probably shouldn't go without these. Of course, this doesn't mean a stall team must run Spikes, or should for that matter, because remember, a stall team just has to not lose. Setting up Spikes is actually considered offensive if it plays a focal point on a team.
With a "balanced" team, you end up losing to teams you could've beaten if you just focused on one aspect. You get outsped by offensive teams and with your lack of true defensive walls, you are faced with a losing proposition. Against a stall team, you lack the offensive weapons necessary to break that stall and ultimately lose the battle as time wears on.
In GSC (but applies to the game in general), there are three categories that every team will ultimately fall under: stall, offensive, and fail. Stall is self-defined. All successful stall teams are extremely similar. The goal with a stall team is to not lose (note: this is the ONLY goal for a successful stall team) and that's accomplished through covering, or making ineffective, as many offensive threats as possible. Here's a short compilation of common threats:
Offensive teams are those that have a chance, akin possibility, of beating any combination of Pokemon through its own offense (doesn't need to rely on freezing or critical hitting the opponent). However, an offensive team doesn't necessarily have to be quick, it just needs a win condition. Teams focused on Spikes can be considered offensive teams, but these teams should not be mistaken for just some poor stall team with Spikes splashed in. These teams should run a Ghost-type or Pursuit user as well as a Spikes user (obviously). It also MUST have Pokemon capable of abusing Spikes through indirect/direct methods of forcing switches (Raikou and Skarmory are the best at it, Suicune a distant 3rd). If the team relies on Toxic, then it should have Toxic on different Pokemon so that it maximizes status spread. This means you must be able to Toxic different stuff; Toxic on Tyranitar and Snorlax is the same as running Toxic on one because they see the same switch-in: Suicune. It's also a good idea to run ample offensive Pokemon to force enough switches for Spikes/Toxic to make a difference. Offensive teams are the hardest teams to build, not that they have any competition though.
A fail team is either a defensive team that lacks both the longevity and the pure walling power of that of stall teams, or an offensive minded team that simply lacks the correct weapons to break through stall. The majority of teams you see will fall under this category. I'll even go as far as to say the classic Celia team falls under offense (Spiking) gearing towards fail (slow as hell). A pure stall team can last just as long as him/her, and he/she's got a huge mixed sweeper weakness in addition to DrumLax.
Stall teams dominate(d) GSC, no one can argue with that. It's pretty much always been true. Why? They're easy to play and yield the maximum results with minimal prediction and skill (hey its Jolteon, go Raikou; hey its Snorlax, go Skarmory; hey its something that cant hurt me, use Toxic; etc). To help its case even further, most teams were built with the "balanced" (fail) philosophy in mind. A player's thinking often goes down like this: well this team has a glaring weakness to Pokemon A, so I'll throw in Pokemon B with moves C and D to cover; Pokemon E might give me some problems, so I'll just replace move D with move G; throw on a Spikes user, Snorlax, and a phazer and I'm done. These teams almost always lacked the offense to break stall, and because it started off as some offensive team, it also lacks the defense to outlast true stall teams. These teams often resort to try and out-stall the stall teams, which resulted in the classic generalization of all GSC as being just that: stall.
But just stand back and think for a second: what is the mindset of stall teams? To not lose. Rather than trying to win, their entire goal is just to "not lose." Fail. So how could a team that doesn't even have a plan to winning possibly win, and with such frequency? The answer: stall teams generally face other stall/fail teams that also don't have any method of winning, just not losing. Because the stall team is dedicated to not losing, and the fail team is just partially dedicated to not losing, the stall team wins in not losing. The truth is, a stall team can never win in its own right. You can beat yourself by simply not executing your offense correctly (that's assuming your team doesn't fall under the fail category), or you can be outskilled in terms of prediction, but you can never flat-out lose to a stall team.
With that being said, a stall team played aggressively is extremely dangerous in that it minimizes the time the offensive team has to execute its offense (or in other terms, when they control the tempo). Along the same lines, if you're playing stall slowly against the offensive team (e.g. setting up Toxic/Spikes/other fail moves), all you're doing is giving the offense a chance to set up (in a general sense) and extending the time to which offensive teams have control.
Now here's a compilation of random posts I made that I deem somewhat useful:
On the topic of creativity
On people throwing Spikes users on the team "just to annoy"
On controlling tempo being the most important aspect of battling
In Sir Chris's epic topic
To sum up his post:
RestTalk Suicune + RestTalk Raikou walls every (used) non-setup sweeper (Dragonite, Marowak, Nidoking, Rhydon, Machamp, Tyranitar, Electabuzz, Mixlax, Gengar, Jynx, etc) aka mixed sweeper, bar luck and Belly Drum Snorlax, and luck beats everything anyways. Also beats old stuff like Swords Dance Tentacruel and Belly Drum Charizard. Growl Miltank can generally wall every Curse user. Swords Dance users are generally frail and/or lack recovery and die to Spikes/whatever eventually. GSC offense gets the short end of the stick.
Now I'm not an advocate of stall teams; they're the best display of a lack of skill in every metagame. What is the purpose of "stall" teams? To not lose. That's a pretty weak objective for a team. That means, they don't even attempt to actually "win," but merely not "lose." A series of systematic A-B switches shows no skill whatsoever.
Stall teams are built in a way that it attempts to cover most of the metagame sets and attempt to wall them with superior stats or playing types; this is basic knowledge. Using mixed sweepers basically lets you claw at every part of that defense, harder in some areas. That's great on paper, but "jack of all trades, master of none." It's better to focus on one part of their defense and continually claw at it and weaken it. Forget the other 5, focus on that one Pokemon because once it goes down, the team is no longer a stall team to you. The first thing that should come to mind at this point is Explosion.
Working on a bait-kill concept, you should be looking towards Exploders that can bait out that specific "wall" and kill it. Using the examples in the first paragraph, if Raikou is giving your Tentacruel/Dragonite problems, use Gengar to bait-Explode it. Suicune a problem? Use Curse Steelix. Zapdos? Use Explosion Exeggutor. Starmie? Use Explosion Cloyster. And so on. Of course, there's always some risk involved, namely a random switch out.
DrumLax ran alongside a cleric is another great combo. "Turbo Drumming" beats Skarmory. Set up Belly Drum, attack, heal with Rest, switch out and use Heal Bell and repeat. Skarmory can't keep up, unless they run a cleric and follow your pattern. This is where a 13 DV Sing Blissey comes in (208 doesn't outspeed anything important anyway). It lets you sleep the opposing cleric, thus ending all hopes of keeping up, unless your opponent runs two clerics.
My favorite stallbreaker by far is Vaporeon with Acid Armor, Growth, Surf, Rest. This was a set I ran back in '04, when Growth Jolteon and Espeon were banned. Not sure how original it is, but I've never seen its use prior to my own, so I guess I can put a claim on it. Clear all Electric-types from the opposing team and it sweeps. Not even DrumLax can switch into it. It outpaces CurseLax for the kill and I run one of my teams around setting it up. GSC is a physical dominated game and subsequently, stall teams generally lack coverage on the special side. Growth users in general, but they're all pretty fragile except for Vaporeon.
Belly Drum users in general can break stall teams (but for all intents and purposes, there are only two plausible Belly Drum users: Snorlax and Clefable). They set up much too quickly for stall teams. This isn't 100% reliable, but it's one of the better options.
Just a few specifics, but you should get the concept from the first [important] paragraph of my rant (the one outlining the structure of stall teams). Mixed sweepers being able to break stall teams are just misconceptions as of one month ago.
On people thinking mixed sweepers alone > stall teams
Tips for people coming from newer generations
The clarification: GSC can be any speed you want it to be provided you're good enough. GSC gets its stall reputation because few players know how to execute their offense in a defensive metagame. Stall isn't really stall unless both players are helpless versus each other. You're not getting 20-30 turn wins, where it consists of "OK you die. Now I die. OK you die. My turn". But "stall" isn't defined in length; to stall by definition is to postpone, akin, postpone the inevitable (loss).
Stall teams, bad ones: if you're running one, and you run into another player running one, then don't complain if battles take forever. You brought this on yourself. In any generation, if you're using 6 Blissey vs 6 Blissey, wouldn't it take forever? So if you decide to run your Suicune, Raikou, Skarmory, Miltank, Cloyster, CurseLax godsex of a team (circa 2002-2008), and you run into the same team, then it's your fault if it ends up being a "draw". Quick fix: run Belly Drum Snorlax. There's no hard counters to that.
Get better offense. Strategize. This game isn't about slapping a Choice Band on everything and getting quad-digit Attack and Speed stats and just winning. It's a chess match. You improve your position with each attack, until there's an opening to go for it. Stuff crumbles from there. This isn't choose the attack with the highest Base Power and go.
Prediction counts for something. One layer of Spikes is all GSC players need in order to capitalize on opponents (Rapid Spin is the sole reason Starmie has been OU for the past decade). Prediction has all but fallen off the map for some reason. If you're in a mismatch, switch. You don't leave it in to die, just so you could bring out something different, only to have your opponent switch. The Pokemon don't run around with 700 Attack and 500 Speed in this generation. You chip at stuff; you have to know when to give up, and when to go for a 10% status, a 25% effect, a 30% status, a 50% move, a 70% move, etc. Know when to back off.
Foresight. Prediction is 1-3 turns ahead. Foresight is 10-30, even 50, 100 turns ahead. It's the long term goal. You've got to have a plan in your head, or you're playing blindly, and chances are, it'll never end. Hence, stall. Hence, most everyone who's ever played GSC says it's stall. Hence, when you watch a battle, you'd know if something is going on, or if two players are just clueless. Have foresight, not foreskin.
There's "luck", then there's "luck" (doesn't work as well online, I know). There's luck that just happens, and luck that you play for. If I'm attacking you over and over, I'm bound to get that critical hit or freeze or whatever. So don't complain when it happens.
Switch. There's no revenge killing here. You're working for your kills. On the other hand, switch less and punish your opponent's switches. Switch more, yet switch less; does that make sense?
CurseLax. Good defense, mediocre offense. It's really only neutralized in GSC, but there's so many Pokemon that can do it. And really, Snorlax is too important of a slot to just throw away if you're trying to break defenses.
SkarmBliss. Get mixed offense.
Sleep Talk. It hits Rest. Get over it. Learn to play around it. And believe me, sometimes hitting Rest over and over again is the last thing you want as your opponent's fishing for that Special Defense drop, or critical hit Cross Chop.
Rest. 60-70% of GSC Pokemon run it. Don't complain. Instead run more strategical offense. Maybe you can't get away with "not very effective" attacks doing 80% in this generation. Learn to play around Rest.
Leftovers. Yes, it's common.
Hitting for super effective damage is irrelevant, look at the actual damages. 38% is a good value for "significant" damage. 30% is a good value for "relevant damage". Anything lower is chip damage.
Sacrificing for an extra attack is nice, but that's new generation thinking unless you have formulated a clear plan to victory. That extra attack, unless it's GUARANTEED to mean something, isn't worth the death almost all the time.
A far more common scenario is the act of sacrificing something to keep hope alive. e.g. Charizard comes in on your Steelix. If you stay in, you're dead if it Fire Blasts. If you switch out, you're dead if it Belly Drums (assuming Charizard was in a position to sweep). Sometimes it's better to cut your losses than to bet everything on a prediction, unless you're that sure about yourself.
One key question to ask yourself repeatedly is: "SO WHAT?" Unless you have an appropriate response, maybe it's better to rethink your moves. Say you have Suicune active vs Marowak, and you pretty much know for sure it's switching out, what do you do? Do you hit the switch with Surf, Toxic it? In either scenarios, ask yourself: "SO WHAT?" So what if you Surf their Snorlax or Raikou? So what if you Toxic them? If there's no legitimate answer, then maybe it's best to improve your position in another way. Maybe switch in Machamp, or your own Snorlax, or Nidoking.
Quick tips:
Don't let stuff die for no reason.
Don't save stuff you don't need for no reason.