How do you write/set up a combat scene in a group RP?

I don't mean "Arena-styled" if that makes any sense, but more like there's a group of enemies in front of your squad and you have to eliminate them. How does one set that up? Does the GM have to heavily detail their positions and everyone talks OOC before making a collab about it? Would using a map or sorts be worth it? What if the players decide not to do a collab and try to do it in separate posts, would that still work out?

What if there's guns involved (ie modern combat or WW2)? Is is just the same idea just at greater ranges? Speaking of which, what about supporting elements like "off map" artillery or some big magical abilities that alter the battlefield?

And then after all of that, then what if you have enemies littered all over the place a la Dark Souls/Bloodborne style?

Just something I was wondering about for a while now.

I think I'll watch this thread considering that exact scenario is going to come up in not too long with sci-fi big pieces of 20-100 ton machines firing lasers, autocannons and missiles with the players vs NPC's...

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

Collab posts is what I am used to, like this. Sort of necessary in an advanced RP since too many short-posts would be unseemly. Basically, get together with the others and write bits of combat, then string them all into one post that one of the group posts.

I think the best thing to do is talk it over and plan out in the OOC. If you don't, you run the risk of people all going after the same target, plus your writing can be improved by stating what's happening and acknowledging who is going where.

Eg: Char A ran off to fight the east, knowing that Char B would be taking on the west
^^^
That's not autoing, or powerplaying or anything since it was discussed prior, but it's a way to collab without having to write together.

Something to keep in mind as well is the time and strength needed to defeat the opponents. Is it a mob that is being slaughtered? Are there leader characters in amoungst the enemies? What will happen if you break their ranks, will they flee or fight back?

Thinking off topic about it, but tabletop games like Warhammer and 40k are probably the best way to relate as you can have a few hero characters vs a group of orks or tynarids and each race reacts differently to different situations.