So many combatants!

[The Cats]

Ran a pretty big combat last night. Four players and their NPC priest, an adult dragon, 18 merfolk, 6 giant sharks, 4 sea lions and a giant octopus. It was a blast for everyone involved (The players were trying to protect the merfolk so they got clever and turned some nearby sea lions into a ball of bloody carnage, luring the giant sharks away. The dragon was helping the players but had just been freed from a prison so was starving and kept stopping to eat its kills. Then the octopus showed up and they had to stop it from stealing their submarine.) but was pretty taxing for me, had to take a break after to rest my brain after keeping track of all of that.

I used average damage and treated the merfolk as three 'swarms' who made single attack rolls and multiplied their damage by 6 (dropping as they lost members) but it still felt like a hell of a lot. Any tips for running not-quite-mass combat like this?

[Ixidor92]

Couple of things I do when running this many combatants:
-Have characters that aren't players act in groups on the same initiative. I don't know if you already do this or not, but in this case I would have all the sharks act on the same initiative, all the sea lions act on the same initiative, etc. When enemies act in groups like that, it means there are less turns to take, and it's also a little easier for the PC's to see what their goal is.

-If there is direct combat that the PC's are not directly involved in, and it doesn't involve 2 major NPCs, don't necessarily have them move, roll attacks, and roll damage. Instead, just roll a single d20 for each side, and give some modifiers based on the situation (one creature is higher level, one side has more combatants, etc.) and give that battle a general outcome based on how the rolls compare. For example, if one side got an 18 and the other a 12, have the 12-side take some damage and be in a disadvantageous position. This lets you avoid long periods of time where the PC's aren't interacting with something on the board

-It's fine for creatures to run away: This is a general rule for me, but especially in a mass-combat. When one side realizes they're losing, a number of the peons will probably flee the battlefield because they like living. This works for wild animals, soldier fodder like goblins, or anything along those lines.

Those are just a couple of things I do when a GIANT combat encounter rolls around. Hope they can help

[Armored Walrus]

-If there is direct combat that the PC's are not directly involved in, and it doesn't involve 2 major NPCs, don't necessarily have them move, roll attacks, and roll damage. Instead, just roll a single d20 for each side, and give some modifiers based on the situation (one creature is higher level, one side has more combatants, etc.) and give that battle a general outcome based on how the rolls compare. For example, if one side got an 18 and the other a 12, have the 12-side take some damage and be in a disadvantageous position. This lets you avoid long periods of time where the PC's aren't interacting with something on the board

I'll double down on this and say that if the players aren't directly involved in it, that fight is pure narrative and no dice are rolled. There's no reason the camera has to focus on what the red shirts are doing, other than once in awhile we notice one or the other of them dying. When the PCs are ready to face whatever the red shirts are occupying, they finish that fight and come back at the PCs, weakened by whatever amount you decide is appropriate.