When you've spent a lot of page time developing a large cast, and you then have a big wacky scene in which they're all involved, it's natural to want to show the reader what all of them are doing. One hopes that they're now interested in the characters and will like to see what part each of them has in this wild scene - which could be a party or other gathering, but is probably most often a fight.
The good news is that you can do this, and do it well. You can describe what everyone - or most everyone, at least - is doing. You just have to avoid the pitfall of making it seem like that's what you're doing.
To go broader for a moment: a common cause of problematic writing is what you might call Author Peekaboo. You're reading along, and the story is moving based on what the characters want or need to do. You're looking at the characters. Then - peekaboo! - a scene that belongs to the author rather than the characters. Basically, a scene is a problem if it seems to the reader like you put it in because you wanted to, whether to push the plot somewhere or, as is usually the case with free-for-all scenes, for awesomeness' sake. Yes, some people will cynically pick out the "plot reason" for even very reasonable story developments, but if they are reasonable story developments, even these people won't likely be bothered. And while I hope you enjoy your writing, you don't want readers thinking, "Boy, the author sure liked this scene." Readers should be seeing your characters, not you.
Now, to go narrower: the way to avoid this in free-for-all scenes is to make sure you describe only what your POV character would actually be paying attention to. Assuming s/he is not a complete spectator to this event - which, by the way, would be really annoying to read - then s/he simply will not be able to take in everything that is happening at once.
When an author writes a long passage of -
Celia and Derek crashed through the hall on the back of Celia's horse, swords swinging, scattering the invaders. Emma seemed to have finally got the hang of the fireball spell, and was tossing flames in all directions from the shelter of an overturned table. Francis leapt onto another table, kicked a wineglass into an invader's face, and ran down the tabletop, lashing out with both ends of his spear.
- usually much, much longer and more detailed, and often detailing the actions of over a dozen characters -
- this is what is in the author's head:
"Hooray! Plot-relevant action combined with character development! Celia and Derek are sharing a horse after I've pushed them together for two hundred pages/three volumes of the series! Bumbling Emma makes good! Francis looks super-hot, leading nicely into his first kiss with Padma Protagonist after the battle!"
This is what is in the reader's head, at least if the reader is me:
"What is Padma doing? Is she just standing there watching all this? How come no one is attacking her? How come she's not helping? Why am I reading from the viewpoint of someone so useless, anyway?"
(The above passage should set off alarms regardless, because you can't tell whose point of view it's in at all.)
So you want to focus on what your protagonist is immediately paying attention to, and this should be related to what she is immediately doing, which should really be SOMETHING. A great way to do this and still show what all your lovely characters are up to is to make your protagonist physically move through the fight. Now being a great time to throw in this entry's obligatory Harry Potter reference, I'll mention the big battle at the end of Deathly Hallows. Even here, I found events a tiny bit list-y at times, but mostly, the scene works very well, and it's because Harry has his own super-important goal. He's moving through the battle, and we mostly glimpse only the pieces of it that get in his way or distract them by specifically endangering one of his bestest best friends. This is a good way to operate. Keep your protagonist's focus narrow, the way it would actually be in a big crazy life-and-death struggle.
So, maybe this:
Padma drew her sword, only to leap aside at Derek's warning shout. He and Celia thundered past on Celia's horse, swords swinging. As the invaders scattered, Padma seized her chance, charging across the hall in her friends' wake. Fifty feet to the barred door that led to the dungeons . . . forty feet . . . The invaders regrouped in front of her, snarling, their archers taking aim. Padma threw herself behind an overturned table.
"Oh Padma, it's only you!" Emma crouched beside her, a sputtering ball of fire in one hand.
"I thought you couldn't do those!" said Padma as Emma lobbed the fireball over the table.
"So did I. Oh! What is Francis doing?"
Francis! Padma risked a look around just as Francis leapt onto a table, kicking a wineglass into an invader's face.
Padma shot a glance at the dungeon door. This diversion could be just what she needed. But what about Francis? He was now running down the tabletop, lashing out with both ends of his spear. What if something happened to him?
(You know, before Padma can get to the dungeon and release the giant monster fox that will snarf the invaders, because they're all extremely violent fieldmice.)
The second passage has all the same info as the first one - indeed, more details. Unfortunately, the style of the first passage appears all too often. Just like any other scene, free-for-alls work best when you take the important stuff that's happening and take your POV character and SMUSH THEM TOGETHER.