Depending on the kind of player and/or game master you are you'll likely either love min/maxing or find it very disruptive to the game. Whichever it is if you're reading this guide you're probably looking for ways to deal with it, either in favor of min/maxing or against it.
In this guide I'll mostly cover ways to counter min/maxing, but I also cover a few ways to simply make it part of the game, which in some cases is just there by default.
Min/maxing is essentially creating a highly specialized character by putting all your available points into one or two skills and possibly even forgoing other skills and talents to further increase these main two.
There's lesser forms of min/maxing as well, but it always involves maximizing certain aspects of your character to make them as powerful as possible.
You might think this isn't a bad thing and it definitely doesn't have to be. Some games are designed to be played like this, in other cases it simply makes sense for your character to be the best they can be in a certain field. However, in some cases it can break the the game or simply disrupts the fun for others. If your character ends up being an undefeatable character while the rest of your party is weak in comparison it usually becomes a one person show, which isn't very enjoyable and usually not really the point of a multiplayer game.
Let's talk a little about game systems. In some game systems min/maxing is definitely the way to go, it's simply the way the game is designed, whether intentionally or not. If you don't min/max in these games you'll likely just end up playing the game in a subpar way, but if you're having fun that doesn't matter at all.
But my point is that if you're bothered by min/maxing you'll first have to take a good look at the game system you're playing. If it encourages min/maxing there's usually little point in trying to fight it. Instead you either go with the flow or change systems.
The easiest way to deal with min/maxed characters? Increase the difficulty of whatever they're up against. Does the tank have armor that can't be penetrated by simple swords? All monsters now have better swords and stronger swinging arms. It's literally that easy. By removing the advantage they have over a regular character you essentially turn them into regular characters again by comparison. It's not a very creative solution and I will delve into other solutions further down, but it's definitely the easiest and a very effective one.
But what if you only have a few min/maxers in the group? Making the monsters stronger would mean the other characters would get destroyed. Well, in this case you're going to have to be more creative. Making the monsters stronger in certain areas could definitely help, but it'll often come across like you're targeting a specific player, which, let's be honest, you kind of are at this point.
Most games rely on teamwork and as such you often have characters specialized in certain tasks. This alone tends to lead to at least some min/maxing, but when one player min/maxes more than others it can ruin it for those who don't. One way of dealing with this is by allowing everybody to min/max and by making sure those who aren't good at it are helped by those who are.
By involving everybody you even out the playing field. Min/maxers get to enjoy figuring out how to make their character as powerful as possible, the game master will be able to more easily balance encounters and those who don't usually min/max will at least not feel like their characters don't have anything to do or are insignificant compared to the min/maxed characters.
Alternatively you can allow characters to aid each other on skill checks, but only if they have at least an average level in that skill. If working together is more advantageous than individually specializing in certain tasks it definitely encourages min/maxers to min/max cooperative play.
Specialization is great, but over-specialization can be a character's or even a team's downfall. If your character is specialized in stealth and assassination you might be rendered useless when you find yourself in a situation where you cannot stealth. If you're a great barbarian who can slash opponents in half in one or two hits you'll find yourself struggling when your opponents only use ranged weapons and manage to keep their distance.
This can go a step further too. If you're specialized in two skills and perhaps even went as far as forgoing other skills you might end up putting your group at risk in some situations. If you're not very good at being stealthy your entire party could be discovered because of you.
If you're the game master you can mention these elements during character creation. When players know there will be situations where having a low score in something will potentially mess up their plans they're more inclined to balance their characters a little more.
Another way of limiting min/maxing is by simply changing the rules. If you're using a point allocation system that relies on buying points simply increase the cost of the higher levels. If you're using a predefined set of skill levels simply change that. This is a very tricky method of trying to prevent min/maxing though, as in many cases it can come across as you being too harsh on the players. You could argue this'll make things more challenging and thus more fun, but to many players the character creation process is a lot of fun in and of itself, so limiting their options would take away from this.
A far better way of changing the rules is by simply giving the players bonus points. Maybe if at least 80% of their skills are at least average they get to spend another point in a specific skill, either primary or other depending on your choice. You could even allow them to spend bonus points on secondary skills and traits next to their normal point pool, which could further character development and just makes for more well rounded characters in the long run without completely unbalancing the game to the point of making it unplayable.
Basing skills based on your character's backstory is something I personally do as a player, but it could be something you'd require your players to do as well. It usually leads to more balanced characters, but some players might simply invent a backstory that'll match their min/maxed points. Rules could be set in place to prevent this, but ultimately this tends to just add more frustrations than it's worth.
A far more fun way of dealing with min/maxers is to make them role play their stats. It'll create interesting characters and fun and memorable game moments, but if you're looking to avoid min/maxing this is definitely not the way to go. In some cases it just makes people min/max even more as it can be a lot of fun to play a character who's incredibly weak or lacks a basic intelligence level.