What CR would this level 5 wizard "mini-boss" be?

Question

I'm about to start the first session of a big campaign tomorrow night. I have three experienced players and one newbie and I've got a big opener planned. The way things will most likely go in the first session will involve a lot of small fights without a short or long rest between them. The party is starting off at level 3. I am going to be forcing them into one CR 2.5 encounter at the start of the campaign which they should win fairly easily but be worn down a bit afterward.

This level 5 wizard is planned to be a sort of "mini-boss" encounter to slow them down in a chase (that is, if they even chose to participate in the chase at all). So he's not meant to be a huge challenge, but I don't want them to blow through him either. My questions are as follows.

• What CR is this wizard?
• Should I make him stronger or weaker?

The Wizard:

Race: Breton (homebrew race for the campaign. Stats very similar to full-blood elf in the base game)

Level: 5

Ability Scores:
STR: 9
DEX: 13
CON: 14
INT: 18
WIS: 12
CHA: 10

Saving throw modifiers:
STR: -1
DEX: 1
CON: 2
INT: 7
WIS: 4
CHA: 0

Health: 30 (8+(4d6+8)) (he will buff himself with false life)
AC: 11 (no armor) 14 (with mage armor)

Spells: Cantrips: Fire bolt, acid splash
Level 1: Mage armor, false life, ray of sickness, magic missile
Level 2: Magic weapon, shatter
Level 3: Counterspell, lightning bolt

Attack: Melee: 4 to hit. 1d4+1 (dagger)
Spell attack: +7 to hit; spell save DC of 15

Feats:
Natural Ward: Damage taken from magic spells is reduced by 1d6 with a minimum of 1.
Grim Harvest

The reason I'm finding this hard to figure out instead of just using the calculation formula in the book is because of two things:

1. His damage output will change drastically based on what point of the battle it is and how many spell slots he has.
2. His Natural Ward feat potentially makes him far tankier than his stats imply because half the party relies on magic damage.

Answer 1

Don't use CR

I don't think CR is going to solve your problems.

It would be great if you could plug in all the info into a calculator and pull out a number which will tell you how challenging the fight will be. And to be fair, there are a lot of very good CR calculators around.

However, the problem I have with them is that CR isn't very good from the start. CR is a rough approximation, but even if you calculated the CR perfectly, you still need to examine the situation more closely.

Asymmetrical Balance

One of the big problems you are going to have, is that NPCs are not PCs. PCs have incredible damage, but not much HP. Your wizard is the same, they have very little survivability, but a lot of damage. That's not the way NPCs should be built.

Take a quick look at your wizard; 30hp + False Life (lvl 3 slot: 2.5 + 4 + 10 = 16.5) = 47ish hp, 14AC. That's not a lot. Many level 3 classes will be dealing 1/3 to 1/2 that much damage on average. 14AC isn't a challenge for level 3 martial classes to hit either. A character could 1 hit that wizard with a luck roll and a crit.

And you have 3 lvl 3 PCs beating on this wizard, every turn. It's likely the wizard will be down before their initiative comes up, heck it's likely that one player won't even get a turn.

Difficult single enemies are easy to beat

Any time a monster is solo, it's vastly easier to beat. Lower level PCs can beat it down with their action economy. There's not much way around this, that's why stronger monsters have legendary actions.

So, what to do instead?

Firstly, don't use PC levels for an NPC. Pick the spells and features you want, and attach it to an existing monster of the right CR.

Secondly, make sure there are always minions in the fight. 1v3 isn't fair. 5v3 isn't fair either, but it's a lot more fun if the players win when the odds are against them! Even if they are only CR 1/4 skeletons, being outnumbered helps with the action economy, and it's fun.

Thirdly, play test. Yes I know, it's a pain, it would be great if you can just plug it all into a formula, right? But play testing will give you a much better understanding. There is no way to accurately predict how your players will react, but using their PCs to test is the closest you can get. It's not as hard as you may think. If you have their character sheets it's pretty straight forward.

Or, like I do, throw all that away

I used to play test, to make sure everything went smoothly. But now I don't. It doesn't matter if things do not go smoothly, actually I personally prefer it. I'm not saying you should throw vastly stronger things at the players, but a little too strong is fine.

If players run in and steamroll your wizard? So what. There are other enemies in the game, don't sweat it.

If players struggle with the wizard, losing a lot of hp and resources? Again, so what. It's up to them to manage their resources, and handle fights and pacing.

I find this works a lot better. The story emerges from the players actions instead of from my head. This makes them happier since their actions are the story, and me happier because I don't have to worry making a good story for the players. This is part of an overall player-driven philosophy to running games. I'm not going to try turn you away from narrative games in one swoop, but if you think about facilitating your players actions instead of dictating them, then you may find unbalanced encounters to be less of a problem.

Answer 2

NPC's or indeed other Characters are not really compatible with the whole CR rating system in 5e, as companion stat blocks don't have their HD and CR on equal footing. So, it is hard to say. A rough guestimate on my part would be CR 2 at most (again, depending on what school that mage is (for not all are equal in battle), which spells (s)he has prepared and available (spells not being equal either).

For a level 1 party, this could be a speed bump or a tpk (depending on which spells are used).

Looking over your spells and tactical gameplay on the players' part in mind, he could potentially kill one PC and die in the second round. That is if he wins initiative. If not, he's as good as dead by the end of round 1.