I'm working on an action-adventure game, and I was considering adding a co-op option. What I'm trying to decide is if when another player joins the game, if the game should get harder. On the one hand, I don't want the game to just get easier. On the other hand, I don't want to break continuity by making enemies suddenly stronger, eg I was able to take this down it two hits before, why does it take four all of the sudden.

I do not want there to be an "AI co-op" that comes with you in single player, I think that would make the game worse since it would detract from player agency.

How can I handle this? How can I keep the game challenging when a second player joins in?

  • \$\begingroup\$ I know that Diablo III does this: when more players join the game, the monsters are harder to kill. One reason for this it to make players play together. \$\endgroup\$ – Vaillancourt Mar 22 '19 at 15:48
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    \$\begingroup\$ Also, what do you expect from the coop experience? \$\endgroup\$ – Vaillancourt Mar 22 '19 at 15:53
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    \$\begingroup\$ This looks to me like a judgement call you'll need to make. Different choices are right for different games and their creative intentions. On the subject of your game's creative direction, you're more of an expert than we are. If you decide that you do want to increase the difficulty and need help designing the mechanics of how, that's something we can help you with. But determining whether to do this is up to you. \$\endgroup\$ – DMGregory Mar 22 '19 at 15:55
  • \$\begingroup\$ @AlexandreVaillancourt I expect from the coop experience basically the same thing as single player, but for two people to enjoy it together and work together. \$\endgroup\$ – rtpax Mar 22 '19 at 16:09
  • \$\begingroup\$ You might find the D&D basic rules for encounter difficulty a good guide to this design question. Basically it's setting an XP level for the encounter based on the character levels and challenge difficulty (easy to deadly), then finding monster(s) that meet that XP level. See dnd.wizards.com/products/tabletop/dm-basic-rules for the details. \$\endgroup\$ – Tim Holt Mar 26 '19 at 18:27

It all depends on the gameplay of your game, but it all comes down to this: If multiple players make the player team stronger, then the enemies have to be stronger as well, to balance this.

Although not a video game, this is how Dungeon and Dragons do it. I don't have the Dungeon Master's Guide with me, but I recall it mentions something like this:

For each player of a specific level, add the XP value together. There are different values for Easy, Medium, Hard and Deadly encounters. Using this you can get the party XP for each difficulty.

When making a monster encounter, add the XP of all monsters and multiply it with some specific values, depending on the amount of monsters. So the more monsters on an encounter, the higher the multiplier value. This is the Encounter XP.

Compare the Encounter XP with each difficulty's Party XP. To find if the encounter is Easy, Medium, Hard or Deadly.

This means that more players means more party XP, means that more difficult encounters are now easier. If you didn't do that, all encounters would just get easier, which could potentially make the game easier.

However, this is not a rule. There are games where having more players could make the game more difficult, because maybe all players have to survive, so if even one of them dies, the team dies. So make your choice depending on your game, just make sure the game has roughly the same amount of challenge.

  • \$\begingroup\$ I like the idea of one player dying being like the whole team dying. That is a very natural way to counteract benefits of the added attack power. \$\endgroup\$ – rtpax Mar 22 '19 at 16:10
  • \$\begingroup\$ For more details on this, see dnd.wizards.com/products/tabletop/dm-basic-rules \$\endgroup\$ – Tim Holt Mar 26 '19 at 18:28

First of all, figure out how having multiple players effects the core mechanics of the game if everything stays the same as single player.

  • Your player "team" now has twice as much damage and health, abilities.

  • Your player "team" can now be in two places at once - e.g. easily solving puzzles which may have required an object placed on a switch to open a door.

However, the players now also have a few potential downsides - do they now have to share equipment, boosts, health kits, gold, xp etc? If so, this can make things quite a lot more difficult. You also need to be careful that you keep the two players reasonably balanced in terms of power, or it can be pretty frustrating for one of them.

There are lots and lots (and lots!) of games to take inspiration from here in terms of how to balance this out.

The most basic (and probably boring) is to simply scale the enemies so they have twice as much health and damage.

A more interesting variation is to have more enemies - twice as much may be more than your engine can handle, or difficult in other ways. So you could take some balance of difficulty/numbers to compensate.

Another step forward would be to add new abilities, enemies as well as health/numbers.

Really good co-op games tend to add new puzzles, elements when more players are present - but this requires a lot more work from your side. I'd suggest starting with some simple scaling out of damage/health/numbers to start with (it's the easiest to implement) and then playtest it. Is it more fun? Tweak those proportions around and see. If it still feels lacking, you may need to look at more involved options.


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