Some time ago, a clever individual on the Wizards of the Coast forum posted an intriguing idea of his about working skill challenges into monster statblocks. Unfortunately, I don't recall this fellow's name, but I have used his homebrew mechanics to great effect in my games, and I'd like you to be able to do the same.
The concept here is that rather than simply bludgeoning a monster to death by lowering its hit points, your heroes can sabotage their enemy through more clever means. Imagine cutting the ropes holding the palanquin on an elephant's back, so that the archers mounted on the beast fall to the ground. Or the party's magic users working complex spells to bring the incorporeal demon into the physical world so that the fighter can harm it with his sword. These escapades don't directly injure the enemy, but they prevent it from using its more dangerous abilities and makes the battle much easier for the PC's.
The DM describes the monster in such a way that the PC's are likely to notice a specific weakness (alternatively, the DM can inform the players directly of the weakness if they roll high enough on their Knowledge checks to identify the creature). To take advantage of that weakness, the party shares a skill challenge in the heat of combat. In general, you'll want to assign average to hard DC's for the skill checks. Using one of the Primary skills for the challenge is a Standard Action. Using a Secondary skill is a Minor or Free Action (depending on the skill in question. Its the DM's call). If the skill challenge succeeds, the monster will be prevented from using some of its abilities for the rest of the battle, and might take other penalties or hit point damage as well. If the skill challenge fails, the PC's will have wasted a bunch of actions in the heat of battle.
The PC's are in the savanna on a dark, moonless night, surrounded by a pack of vicious gnolls. The filthy beasts growl and wave their weapons as they advance from beneath the tall grasses, looking frequently at the largest member of the pack for instructions, waiting for her cue to strike.
One of the PC's rolls a History check to see what he knows about gnolls. He gets a 26, which beats the DC 25 check to learn about their secret weakness. The DM tells the player - in addition to everything the Monster Manual says a roll of 26 should teach him about the creatures - that gnoll packs are led by the alpha female, who rules through intimidation and dominance displays; making the alpha appear weak or ridiculous might wreak havoc on their animalistic morale.
The primary skills for this challenge are Intimidate, Bluff, and Acrobatics. Secondary skills are Insight and Nature. The skill challenge has a complexity of 1 (four successes before two failures needed to pass the challenge) and a DC of 20.
The Warden acts first. He marches up to the alpha gnoll and, waving his enormous hammer over his head, he howls in primal fury (Intimidate). The Warden's player rolls a 21 on his Intimidate check; just high enough to make the gnoll forget herself and flinch in fear from his feint. The other gnolls see this as a sign of weakness. That's one success. Unfortunately, that was also his standard action for the round.
Next is the Rogue. He looks at the mildly cowed gnoll and makes an Insight check to see where she could best be attacked from next. He rolls a 23, which allows him to see that the alpha gnoll is off-balance from the Warden's intimidation. He turns to the Monk and tells her this, costing himself a minor action but giving the Monk a +2 bonus to her next skill check.
Its the gnolls' turn now. They close into melee and begin slashing at the heroes with their crude spears and axes. The fighter is flanked, preventing him from getting close to the alpha female...but the monk is not. She darts over to the alpha female and makes an Acrobatics check to confuse the beast with her movements. She rolls a 25; the alpha gnoll tries to slash at her, but is thrown off completely, and ends up banging the handle of her spear against her own paw and yelping in pain. A couple of the other gnolls appear to be chuckling in their hyena-like way.
Its the alpha gnoll's turn next. She attacks the Monk and does some nasty damage. She also uses some of her remaining dignity to give forth a deep howl, the sound of which whips her minions into a frenzy. The PC's need to break up the gnoll pecking order quickly, or this could get deadly.
Next is the Wizard. After casting an area spell to clear the attacking gnolls away from him, his player asks the DM if he can use Arcana to humiliate the alpha. The DM is taken aback, but decides "sure, why not?" The DC for this is going to be higher (25), though, since this is an unlikely skill for the situation. The Wizard spends an Action Point so he can take another Standard Action and tries to use an illusion to make the alpha gnoll appear to be wearing a dressing gown and wig. Against everyone's expectations, his player rolls a natural 20, for a total skill check of 32. The alpha gnoll seems to be wearing foppish formalwear, and the other gnolls are finding her harder to take seriously.
On the next turn, the Warden starts by trying to Intimidate the alpha gnoll again. This time, he only gets a 16, which isn't enough to faze her. He's wasted his standard action screaming and stamping his feet, and needs to spend his Action Point to flank the alpha gnoll and attack her with his hammer.
The Rogue slinks away from the gnoll attacking him (soaking up an opportunity attack in the process) and makes an Acrobatics check as he charges the alpha. He rolls a 22, and manages to make her trip over her own feet and fall facefirst in the mud, getting the muck all over her illusory gown. Four Successes, only one Failure. The gnolls can no longer respect their leader; they all suffer a -2 morale penalty to attack rolls for the rest of the encounter, and the alpha can no longer use her Howl and other leadership abilities. The fight is now a piece of cake for the PC's.
-The skill challenge should be appropriate to the type of monster and the battle conditions. A skill challenge using Arcana and Religion could prevent a mummy from using its Fear and Mummy Rot powers. A skill challenge using Athletics and Dungeoneering could cut off a hobgoblin soldier's full plate and drop his AC into the low teens.
-The skill challenge should have average or hard DC's and a complexity of 1. The idea is that it should be no easier or harder than hitting the monster with conventional attacks.
-Only Elite and Solo monsters should come with skill challenges (normal monsters aren't worth the trouble).
-The PC's should be able to learn about the skill challenge by listening to the DM's description of the monster, or by rolling a high DC on their Knowledge checks to identify the creature. Ideally, both methods of learning about the challenge should be available.
-You should include enough different skills for every party member to be able to contribute. You should also make sure that succeeding on the skill challenge is worth all those actions; make the monster pay dearly for their success.
-To use some skills, the PC must be adjacent to the monster. To use others, there should be a maximum distance of 5 or 10 squares. Use your judgment.
-The skill challenge can only be attempted once.
-Using a Primary skill only requires a minor action, but failure provokes an Opportunity Attack from the monster.
-Instead of penalizing the monster, succeeding at the skill challenge inflicts a massive amount of damage on it. For instance, climbing onto a dragon's back and stabbing it in the head (Athletics, Acrobatics, and Bluff checks required) might cause the beast to lose one quarter of its maximum hit points.
-The skill challenge can be attempted multiple times, but the DC's increase each time.
-Failing the skill challenge makes the monster stronger or heals it.
-Normal, non-elite or solo monsters can have skill challenges associated with them. Beating the skill challenge kills the monster outright.