Building an encounter in 4th Edition requires more than just slapping some level-appropriate monsters together and calling it a success.
We've seen an example of an encounter built around the theme of "grabbing", where focused design made a creature team-up more than the sum of its parts. Here we see the other side of the coin, where theoretically competent enemies are just unable to work together to present a credible threat.
The through-line for the Well of Demons is the gnoll Maldrick Scarmaker. He's keen on re-dedicating the Well to the gnoll deity Yeenoghu, and to that end he's left a bunch of his troops guarding the entrance rooms of the Well while pressing on to the Inner Sanctum with his best men.
This encounter represents the last of Maldrick's "entrance guard"; a gnoll "demonic scourge" stationed at an old shrine to Baphomet. The Scourge has a pet Barlgura in the adjacent chamber, and when combat inevitably ensues, the two will join forces.
I mentioned before that the designers didn't seem to be aware of how Barlguras work. They're heavy hitters with a long reach and weak defences; like any glass cannon they need a competent defender ally in order to do their job. The Scourge isn't a defender; in fact, it's another brute. The two enemies have a lot of damage output between them but no way of staying alive long enough to make use of it.
What's more, the most memorable special ability of both the Scourge and the Barlgura is an ally buff. The Barlgura gives all allies in burst 5 a to-hit buff when it goes bloodied; the Scourge gets a damage bonus when it has at least two adjacent allies, and can give up to two adjacent allies a free melee attack when it manages to bloody an enemy.
As a pair of individual monsters, the Barlgura and the Scourge are totally wasted. They don't have enough allies to maximise their buffs, and barring the possibility of some very lucky rolls they're not going to stay alive long enough to knock anyone into the bloodied range, let alone provide a real threat.
There is a wildcard in the fight. The room also features two tieflings, who are here to trick the gnolls out of some unspecified "items and lore" that the gnolls have found in the Well. They're opportunists, and if the fight starts going badly for the PCs they'll help the gnolls, but it won't, which leaves them sitting on the sidelines.
After the fight, there's an opportunity for the PCs to ally with the tieflings, whose names are Azkelak and Katal. The tieflings are wearing black, are hanging out with gnolls apparently of their own free will, and didn't come to the players' aid during the battle. It's a foolish party who trusts these two (even without the magical lie-detector of a good Insight roll), and indeed the Tieflings will stab the PCs to death in their sleep if given the opportunity.
My players took the uncharacteristically unheroic step of cutting the tieflings down where they stood; a quick search of play write-ups of the Well suggests they're not alone. A long history of traitorous NPCs, starting with Ninaran in Keep on the Shadowfell and continuing on to Terlen Darkseeker in Thunderspire, might leave players rightfully intolerant of suspicious adventurers claiming to be their friends.
The encounter's at least necessary. After turning the occupants of this area into corpses, players are free to take The Book of Wrath Unveiled, a holy text of Baphomet which is one of four items the players will eventually need to enter the Inner Sanctum and confront Maldrick.
 How did the tieflings get past the phalagar and its friends? The gnolls can claim safety in numbers, but surely the tieflings should have been eaten alive?