Look, gnolls aren't a bad monster, but putting them next to demons just doesn't do them any favours.
Unless the DM has been making a lot of use of the random encounter table, the Gnoll Barracks will be the players' first run-in with Maldrick Scarmaker's gnolls. It features a gnoll huntmaster and his pack of hyenas. Gnolls being a kind of unpleasant hyena-man, it's a natural match-up. There's a couple of gnoll brutes thrown in for good measure.
There's nothing wrong with this encounter. It's a pefectly by-the-numbers dungeon scuffle. There's some humanoids who need killing, and the players kill them.
The sin is only that players have been here before. It's an identical set-up to any number of battles both in Keep on the Shadowfell and earlier in Thunderspire Labyrinth. You could replace the gnolls with goblins, hobgoblins or duergar and nobody would notice.
Compared to the memorable and imaginative encounters elsewhere in the Well of Demons, this is the ugly stepsister. Hurry through it; there's better coming.
 Not an improvement as such, as it's strongly suggested in the module itself, but the way to wring the most out of this encounter is by emphasising the flavour. The hyenas aren't just hyenas - they're vicious, hungry animals in an enclosed space. Their howls will echo up and down the stone corridors and be virtually deafening at close range. They should be, really, quite frightening. Of course, that doesn't change the fact that they're level 2 monsters in a level 6 encounter and there's a good chance of them falling over the first time someone breathes on them.