Learn to DM: CR and Action Economy

[Charon]

How do you balance encounters? How does CR relate to player strength? Why do your party of level 10 players mince your CR 12 boss in one round?

Welcome to a short article that will clarify some points about CR and explain the concept of action economy among some other notes.

So first up, let's define action economy. Those level 10 players mince the CR 12 boss because they act four times per turn and he acts once. It's as simple as that. Any fight for the PCs should begin at even action economy or, more likely, with them outnumbered. Bringing the action economy under control is the PC's job. To do this, we use adds, additional monsters that either either start the fight with the boss or turn up during. Instead of throwing a CR 12 boss at those PCs you can throw a CR 8 boss with multiple adds. Now the PCs have to mop up the chaff to bring the action economy under control before they can wail on the boss. We could even lower the CR of the boss to allow us to raise the CR of the adds, making them a viable threat in their own right and not just one-hit mooks to be mopped up by the party's barbarian and his great cleave attack.
Conversely, the same is true. If you throw twelve CR 1s at those PCs it will probably be a tougher fight than the one CR 12 for the same reason, the action economy is stacked against them 3 to 1. Even if they each kill an enemy in round one, they start the next round with the economy stacked against them 2 to 1.
What you need to understand as the DM is that the longer it takes the PCs to get the action economy balanced in their favour, the more damage they will take. A fight where it takes two rounds to even out the economy is a pretty tough fight.

The other thing about CR is that it isn't perfect. Sometimes a monster's CR is not a perfect reflection of its strength. You'll learn by doing so pay attention to the monsters you use and how easily the players deal with them. Sometimes the monster's stats will make it obvious. In 3.5e a good example is the Animated Object. A tiny animated object like a candlestick is CR 1/2. Pitiful. But in 3.5e, objects have hardness and without a magical weapon, most characters won't be able to deal enough damage to bypass that hardness. Based solely on CR, a party of level 2s against 3 of these should be able to win easily but I guarantee, it could be a fatal encounter for them.
Likewise, a hard hitting enemy like a Dire Wolf is CR 3, but as my group found out, facing two of those at level 6 is still a deadly encounter. Why? Because the wolves deal 1d8+10 damage. If they crit that's 2d8+20, enough to instakill two of the three players in my group. It almost happened, the party's rogue taking one hit and ending on -7hp. The takeaway point here is that two of my players are glass cannons so a monster that hits hard is more deadly to them to one that can consistently chip damage while staying alive.

Ok, so how does CR work, in general? A CR 10 enemy is not a balanced encounter for a party of level 10s due to the action economy but what about two 5s?
Well, CR is not additional.
The simple way to do this is the use the Encounter Calculator either by manually working it out using the DMG or using the WOTC calculator online, which is great for 3.5e. If you're running 5e, as always, your job is that much harder but someone has probably made such a calculator online.
Firstly, the average party level of the players is their total levels divided by 4.
A party of four level 10s is 40/4 = 10.
A party of three level 10s is 30/4 = 7.5.
A party of five level 10s is 50/4 = 12.5.
Knowing the average party level will help in balancing the encounters, especially as you now know about balancing the action economy too. Five level 10s not only need to face higher CRs than normal but the action economy has more wiggle room too!

So, CR is a measure of a monster's strength but what we're really interested in balancing is the Encounter Level or EL against the APL (Average Party Level).
The relationship between CR and EL is complex but in its simplest form... for every two points in CR you drop, double the monsters.
So your PCs are level 10.
EL 10 is one CR 10.
EL 10 is two CR 8s.
EL 10 is four CR 6s.
Well hold up. If we have four level 10s... then four CR 6s is the right EL and a balanced action economy!

:O We did it. This will be a balanced fight...
On paper.
But what if the monsters aren't all the same CR?
What EL is a CR 8 and a CR 7?

XD
Look, the real talk here is use an Encounter Level calculator.
For 3.5
https://www.d20srd.org/extras/d20encountercalculator/
For 5e
https://kastark.co.uk/rpgs/encounter-calculator-5th/
The difficult math has been done for you. All you need to do is work it out, then pay close attention to the action economy and the specific abilities of the monsters that could render them stronger/weaker based on party composition.

Now you just have one more thing to consider. The other reason why your PCs are stomping your bosses every single time.

Encounters per day.

So you send the PCs off to fight your boss, fully rested, with a full complement of spells and abilities and they wipe the floor even with the adds and action economy against them.
CR and EL does not indicate that the fight will be tough for the PCs, just that it will be sufficient challenge. The PCs are expected to face multiple challenges per day. Before they fight your big boss, they should fight other groups of enemies first to force them to use up some of their resources.
The key figure in 3.5e is that a balanced encounter should use up 20% of their resources. Meaning the players can handle four encounters per day before needing to rest.
In 5e, PCs are more resilient due to the short resting function and the average is 6 (to 8!) balanced encounters.
Make the players use some stuff before facing your boss! They should be able to handle more than you're probably throwing at them. And if they can't, it's probably because they aren't using their consumables. ¬¬
But that's a whole other matter.

For now, we've discussed how CR isn't perfect, but a reflective indicator of difficulty that we, as a DM, need to check. We've seen how CR doesn't add and is instead formulaic, leading to a complex method to calculate EL.
We've seen how fights should be balanced between EL and APL and how action economy can play a role in those calculations.
Finally we've seen that these stats all assume the players will face 4 (or up to 8!) encounters depending on edition.
Remember, your job is not to kill the players. But pushing them to their limits is fine! The boss fight will be so much more memorable if they don't stomp all over him but have to work for the victory after already been worn down.
Don't worry, you will develop a feel for this the more you DM and don't forget, in a TPK like emergency you are well within your rights to fudge. No one wants a TPK (unless you have *that* sort of crew) so changing a crit to a natty 1 is perfectly allowable within the rules, although see my article on failing forward for ways to make it less cheesy.

Happy DMing.