I think maps are really good for large groups, just to get a scale of exactly how far apart they are or how far away the party is. If you're going to make a map, make sure to note where cover (low walls, trees, columns, etc.) are - especially if guns are involved. Make sure to label the scale of your map too. You don't want your players to be wasting their time trying to hit an enemy that they think is 5 feet away when they're actually 50 feet away.
You could probably lay out the scene in your own IC and then do a turn-based action system. Give your players a certain amount IC time to move/react to things in their own IC posts then address the party's actions as a whole in an IC post after everyone's taken their turn. This way the actions of the player party can move at the same pace as the NPC party. The players could probably collab with this style too, with the players collabing by themselves to make their moves and then you respond. This way the post order is Party>NPC>Party>NPC.
Sorry if that doesn't make sense :x