Help with calculating low level encounter challenge rating

[Falainothiras]

Hello everyone, I've recently started DM'ing again after years and I'm a bit confused about regulating an encounter I've created.

My group is 3 players at level 3, a dual wielding + feat BattleMaster Fighter, Devotion Paladin Dwarf and Evocation Wizard Elf.
CR Group Thresholds: 225/450/775/1200

I've created the encounter of the adventure's climax where the players face the villain, a level 5 GOO Warlock, with a custom pet (gibbering orblet/ custom, credit /u/the_singular_anyone). The room where they fight is difficult terain. The warlock has the slow effect incantation and generally mind affecting spells.
According to the CR rules, this encounter appears to be far above deadly. I'm of the impression it is not.
Your insight is appreciated.

[coredump]

How did you determine the CR of the Warlock?


With only 3 PCs, you need to be really careful. One (or heaven forbid two) unlucky Saves and the party is in real trouble.

[NewDM]

Yep, because its a spell caster it could go either way really. It could totally destroy the characters or they could wipe the floor with it. Its easy to determine the CR of a sack of hit points that deal damage. Its not easy to deal with spells and conditions.

[Falainothiras]

How did you determine the CR of the Warlock?


With only 3 PCs, you need to be really careful. One (or heaven forbid two) unlucky Saves and the party is in real trouble.

That's the thing, I haven't found anywhere online a proper way to calculate the CR of a classed character, I'm guessing because of the different variables. I considered him being CR 5 due to his Hit Die, and because I recall having seen a similar answer in a thread somewhere (pretty much the same situation).
A way to alleviate this seems to be cheating on the go in order to avoid a TPK...

[Falainothiras]

Yep, because its a spell caster it could go either way really. It could totally destroy the characters or they could wipe the floor with it. Its easy to determine the CR of a sack of hit points that deal damage. Its not easy to deal with spells and conditions.

Do you think it's possible to calculate the CR of a spell effect, i.e. Slow? That would really help here. The reason I picked the spells to be mostly mind affecting is because I believe it will reduce the impact they have vs damaging spells.

[Firechanter]

Afaik the guideline for NPCs with class levels is CR ~ 2/3 levels. So a level 6 NPC would be roughly CR4. Fingerwigglers are more dangerous than stickswingers, but of course YMMV.

[NewDM]

Do you think it's possible to calculate the CR of a spell effect, i.e. Slow? That would really help here. The reason I picked the spells to be mostly mind affecting is because I believe it will reduce the impact they have vs damaging spells.

Sure, but a spell caster has 2x-3x as many spells prepared as spell slots. So you don't know what they will cast until combat starts. Not only that, but a spell can have two different ratings based on whether the target saved or not. So Slow for instance does nothing on a successful save. So it would have a CR of 0 if you save. On a failed save you can't use bonus actions. Some classes and characters don't use bonus actions at all, others depend on bonus actions to get up to half their damage. Lowered AC and Dexterity saving throws would increase the effective damage of the creature by 5 points at levels 1-5, 7 points at levels 6-10, 9 points at levels 11-15, and 11 points at levels 16-20. At level 5 losing your extra attack is only about half your damage which is equivalent to raising the monsters AC by 4 points. At level 20 losing 3 of your attacks is losing 1/4 of your damage which is like increasing the monsters AC by 6 points. Halfing the rounds of a spell caster lowers their damage by half, at lower levels this is only worth a few AC points, at higher levels its worth a lot more since some spells can take a creature out instantly.

Altogether you are looking at anywhere from 0 to 4 CR for the Slow spell, which is more dependent on the characters level than the creatures.

[BW022]

My general advice on encounters...

1. Don't put much faith in CRs. Ignore them most of the time. They are at best a rule of thumb. Player skills, luck, tactics, the situation, a mistake during play, forgetting to use an ability, spell selection, etc. and make them all moot in a second.

2. Toss lots of different encounters at the party and see what they have problems with. Maybe write some notes as to how hard or easy different encounters are. Goblins in trees, orc shaman with wolves, rogues attacking in an alley, fight in a tavern, etc. Keep track of what the party might be weak at... swarms, ranged, spell caster, etc. When looking at an encounter...look at your notes and realize that the party is strong/weak vs. that type of combat.

3. Give your party a 'get out of jail' item. Some powerful single-use item which the party can use just incase the encounter is too strong for them. A scroll usually 2-3 levels higher than they can normally case, some serious cure potions, etc. They will likely horde it, but if you really mess up on the encounter... they have the option of saving themselves from a total party kill (TPK).

4. On high-impact or big encounters... it is ok to plan something too powerful for the party. However, just give the bad guy some goal other than to kill the PCs. For example, your warlock could be trying to get to a boat and escape. Maybe he is dealing with some orcs and he wants the PCs to slaughter them while he flees. Maybe he is trying to get them blamed for someone's murder. Maybe he has a deal to leave the PCs for the orc leader? If he has some alternate goal... he can always flee, leave them unconscious, take them prisoner, etc. Or maybe some other NPC comes in and drives the bad guy off. A bad guy who toys with you often makes it far more satisfying when the PCs encounter him again in two or three levels and can finally defeat them.