Alex's feedback is a good place to start. And if you want to get a little bit deeper into the weeds, AW offers you plenty of interesting options.
First thing first, if you have that many people involved, I'd say go ahead and use the peripheral battle moves. They are totally appropriate for a firefight of that size.
In order to facilitate a good firefight, take advantage of one of the other bits of AW advice, which is "draw maps like crazy." Map out the landscape/terrain/setting where the firefight is taking place, and do so in a way that is interesting and lends itself to setting up later moves. As an example, maybe the firefight is taking place in the manufactory, where there's an assembly line where molten metal is being stamped. Crossing from one side of that line to the other might require someone to act under fire, for instance. Or there's heavy smoke coming from a pile of burning tires, so spotting a target without exposing yourself requires you to read a sitch before you can engage. Or access to the storm drains running under the complex lets you move unseen and go aggro on someone the first time you emerge in a new location. Or shooting at anyone taking cover behind the diesel distillation lines requires you to spend one of your hold from any sieze by force roll to "take definite hold of it" to keep from blowing the place sky-high.
Similarly, you can assign armor to certain areas of the map if it's appropriate (i.e. taking cover behind a pile of rubble is armor 2, but climbing up on the gantry leaves you exposed and provides no armor).
Once you've mapped out the setting, determine the locations of the various PCs and NPCs within it. If you're running this using the peripheral battle moves, decide where you are in the countdown. Then just let things unfold as they will.
The battle moves are nice because there is the assumption that everyone is getting shot at pretty much all the time after 9:00. Unless a PC opts to stay the fuck down and manages to pull it off successfully, they are going to end up taking plenty of Harm along the way, which will motivate them. And speaking of Harm, it is important to establish what the PCs are dishing out and what they are facing, as well as how you're going to be tracking it. Are you going to treat all of the NPCs as individuals, or are you going to amalgamate them as a gang? The gang rules mostly deal with how to offset the damage based on the engagement of groups of differing sizes. If your NPC baddies have the kind of 2:1 numerical advantage you mentioned in your previous post, I might offset the damage by a step (meaning they inflict +1 Harm and take -1 Harm). But more important, it means you don't need to track Harm on individual NPCs. Just keep track of how much Harm the PCs inflict collectively and translate that into casualties as you go based on what's happening (more on this in a sec). It is important to keep in mind, however, that "inflicting harm" will usually require a PC to either go aggro or seize by force. Enforcing the rule of "to do it, do it" is important.
When it comes to applying casualties in a gang situation, let's say for sake of argument that the NPCs count as "2-harm small gang 1-armor" and that your PCs are outnumbered 2:1 or more. As such, the NPCs now count as inflicting 3-harm because there are more of them. Further, the NPCs will be taking 1 fewer Harm every time the PCs inflict it. At first blush this seems sort of weird - after all, why should my pistol do less damage if I'm shooting it at a guy who is part of a gang versus shooting it at a guy on his own? The answer, of course, is that 2-harm to a gang is different than 2-harm to a person. 2-harm will kill most unarmored NPCs. 2-harm to a gang is "many injuries, several serious, some fatalities." The reason this is important is because it is up to you as the MC to describe things narratively. The PC who unloads with a 3-harm assault rifle might only be inflicting a single point of Harm to the NPC gang on their action, but if that single point of Harm might take things from "a few injuries, one or two serious, no fatalities" to "many injuries, several serious, some fatalities" it can be a significant contribution. So while the PC is only inflicting 1-harm to the gang, in reality they are injuring and/or killing multiple NPCs with their action. This needs to be narrated appropriately such that the situation unfolds naturally.
Similarly, taking Harm from a gang will be more dangerous if the damage stages up due to size difference. This is reflected by the idea that fire is coming from multiple different sources, it's hard to take cover, and any time you poke your head out someone takes a potshot at it. Make sure to narrate this as well.
Once you have the terrain determined and you know how you're going to track the Harm, the next step is to let the combat unfold naturally through the use of the players 'moves. Most of the time, they'll just be using the basic moves and you will narrate accordingly. So for instance, say that the Gunlugger wants to move from where she is to new position that offers an elevation advantage over the enemy (negating any armor benefit they get from the terrain, for instance). To do so, the MC looks at th map and deems that she must cross open ground, which is acting under fire.
Also keep in mind that the basic moves are used in conjunction with the battle moves. Say that it's 9:00 in the battle countdown and now the PCs are taking concentrated fire. The Gunlugger is again going to make a break for the elevated position, and this time the Battlebabe is going to provide covering fire. All the rolls are made simultaneously, so the Gunlugger doesn't know if the covering fire is successful before she runs out into the open. But the Battlebabe is Cool+3, so chances are good. In this case, both players roll+Cool (the Gunlugger for acting under fire, the Battlebabe for providing covering fire). The Battlebabe gets an 11, which means she is successful - the net result of which is that the Gunlugger only takes incidental fire as she crosses open ground. The Gunlugger gets a 7, and the MC decides that as a "worse outcome" the Gunlugger makes it to the elevated position, but that that position is itself an untenable one (meaning she'll take concentrated fire next turn if she stays there). The MC narrates what the various NPCs are doing and the battle clock advances to 10:00.
Now the Gunlugger is in kind of a shitty position, but she's Hard, so she elects to maintain an untenable position. She easily nets a 10, meaning that for 3 ticks (which is the reso of the battle), this position will only come under incidental fire. The MC narrates this as the Gunlugger putting out such a heinous volume of fire from the elevated position that the NPCs are busier scrambling to find cover than they are shooting back. Meanwhile the Battlebabe sees an opportunity and elects to follow through on the Gunlugger's move. To do this she rolls+Hx with the Gunlugger. These two fight together a lot, so maybe that's +2. She gets a 7, meaning she has created an opportunity, but hasn't followed through on it yet. The MC provides a couple of options, and the player decides that the Battlebabe is taking advantage of the NPCs heads being down to break contact and sneak up behind two of the distracted gangers (though she'll still take incidental fire, some of which might actually be coming from the Gunlugger's wild spraying). The MC narrates what the various NPCs are doing and the battle clock advances to 11:00.
Now that her position is secure, the Gunlugger decides that it's time to straight up blast people, classic seize by force. Note that of the rounds I've described, this is the first time that the PCs will have actually inflicted any actual Harm on the NPCs. She rolls+Hard and nets a 9. She chooses to inflict terrible harm and impress dismay or frighten the enemy. She's using an assault rifle that would ordinarily do 3-harm, +1 for inflicting terrible harm, -1 for gang size offset, -1 for the gang's personal armor - they get no benefit from any terrain-based armor because the Gunlugger is firing from an elevated position, for a total of 2-harm. That means in this tick alone, the Gunlugger has inflicted "many injuries, several serious, some fatalities." A veritable firehose of bullets! If only she'd taken "NOT TO BE FUCKED WITH!"
For her part, the Battlebabe has yet to capitalize on her opportunity, so the MC gives her the following option - make a successful act under fire to cover ground quickly and quietly and if successful make an immediate go aggro roll. Rolling+Cool she gets only a 7, so the MC narrates that she has stepped on a half-buried piece of sheet-metal that makes a loud noise, alerting her would-be targets. He offers her this ugly choice - freeze where she is (out of line of sight) and forfeit any chance to do damage this tick, or go balls-to-the-wall, cover the remaining ground as fast as she can, and exchange fire in seize by force. She's Hard-2 and unlikely to come out on top of that exchange, so she opts to stay put (though she's still subject to taking incidental fire).
The clock advances to 12:00, which is the end of the battle. Because the Gunlugger has inflicted significant casualties and frightened the enemy, the MC narrates that the NPC gang starts to withdraw, ceding the battlefield to the PCs. Depending on the presence/absence of the NPC gang's leader and how much total harm has been inflicted on the gang, that withdrawal might be relatively orderly or it might be a rout.
When it's your turn as MC to talk about what the NPCs are doing, it's mostly going to be things that change their position relative to the PCs or that change what's going on in the battlefield. So think of it not so much in terms of "NPCs A and D are shooting at you, take X-harm," because the battle moves dictate that incidental or concentrated fire is happening regardless. Instead, think of it in terms of "NPCs A and D are scrambling up to the top of the junk pile. That's going to give them clear line-of-sight to the loading dock, making the Driver's position untenable next tick." Or "NPCs A and D are laying down withering suppressive fire on your position. Anything you do besides stay the fuck down is going to be acting under fire."
But as always, the key is in the narration. Translate the raw rolls into something meaningful in the fiction, and when you describe things do it in a visceral way. Play on all of the senses - the boom of explosions, the rattling vibration of bullets hitting your cover, the acrid smells of burning tires and cordite, the tinkling of spent brass as it falls through the grating of the catwalk, the thick red smear left behind on the wall by a combatant as he slumps lifelessly to the ground.