build-based combat system and you are getting bored with the combats. Your trip guy is tripping people left and right. Your disarm guy is making sure no enemy has a weapon. Your tumbler is weaving in and out of combat, flanking everyone. This is great, but there isn't much variation, and not a whole lot of challenge either. What do you do?
Well, you could make the enemies tougher, more dangerous and even more numerous to counter the awesome prowess of your PCs. This is fine, but it doesn't do anything to actually get the group out its combat rut. The fights will be longer, and maybe rougher, but no more varied.
A lot of GMs will rely on a tactic I like to call negation to challenge their uber-build PCs. Let's take our trip PC. This guy has trip weapons (whips, chains) and has a bunch of feats that allow him to trip opponents effectively and then either smash them when they are down, or get extra attacks against them when they get up. Against normal opponents, he is devastatingly effective because he either forces opponents to fight when prone (with him getting a bonus, or the enemy taking penalties, or both), or get up and take attacks of opportunity.
Not exactly. If you look at it from the player's point of view, he spent a ton of character experience to become the master of that single maneuver -- probably to the detriment of another aspect of his abilities. By negating his go-to maneuver, you have invalidated all of the experience he spent. For instance, in D&D, some of those feat builds take a dozen levels or more to complete. After spending all of that time, it is not unreasonable for the player to expect that his character would be awesome.
So, if tougher enemies are out and negation is out, what else is there? Well here's the key, both of these tactics can be extremely useful when used thoughtfully and in moderation. As frustrating as some of these builds can be, the goal is not to defeat them, or punish the players for using the game rules effectively. The goal is to make the battles interesting and to get the PCs out of the habit of using the same handful of feats and spells over and over.
A great way to do this is to look at the entire group of PCs, mix up your enemies, and focus on the matchups that you present. Don't toss a bunch of dull, uniform encounters at your PCs and expect them to react with anything but the tried and true tactics. ("Oh, 10 goblins, each with a short sword and shield again?") Look at the PCs and try to determine what kinds of interesting things they could do in a fight and write your encounters to allow them to do those things.
So, I make one of the bandits a big, hulking brute. He is a good fighter and does a lot of damage. This makes him very dangerous to engage head on, but a perfect candidate for the trip master. If the brute stays on his feet and gets a few shots off, someone might get hurt, but if he can be rendered prone, he can be countered and killed.
Another bandit mirrors the PC mobility build and can quickly get around the fighters to flank, or strike at the cleric. He is also very agile, hard to hit and can't be tripped. This guy could cause serious problems by either flanking with the brute, or harassing the cleric or the mage. The spell casters have the key to defeat him though, either by casting area effect spells that auto-hit, or by focusing on his low willpower.
Another bandit has a thrown net to go along with his normal weapons. The net will entangle whoever it hits, making movement, attack and spell casting impossible. Even worse is that once he entangles someone, he has no problem with stabbing the defenseless hero until he is dead. This guy is dangerous to pretty much everyone. No one PC directly counters him. The group will have to make sure that if he manages to entangle someone that help is forthcoming instantly.
You aren't directly negating anyone's cool powers, but if they end up with the wrong matchup, it could go poorly for them. The mage does not want to be captured in the net. The cleric does not want to face the arrows of the skirmishers. The rogue does not want to get hit by the brute. The fighter does not want to be flanked by the mobile bandit.
On the other hand, everyone has something to do or a matchup that is favorable to them. The trip fighter negates the brute. The rogue kills the skirmishers. The cleric and the mage work together to take out the net thrower and keep the mobile bandit at bay.
These battles have a wonderful tendency to go off-script, forcing the players to react and make interesting choices. What happens when the trip fighter learns that he is up against an acrobat that tumbles away from his chain attacks? What happens when the brute gets into the back line? What if the rogue gets tangled in the net? All of a sudden, position and teamwork become critically important -- not just which powers you picked last time you leveled up. You have effectively dealt with your uber-builds without making them any less powerful or cool.
All with about 10 more minutes of prep... try it next time you are bored with your combats, frustrated with an effective character build, or tempted to toss down yet another boring encounter.