Is there a guide on how to create a monster as a solo combat challenge for a PC?

Question

I have read all about how to create an encounter or "boss" monster for a group of PCs to defeat based on challenge rating, but I have not seen any guidance on how to create/modify a monster to make it a challenge of a certain rating for a one-on-one fight with a PC.

Essentially I want a PC (level 5) to be forced to fight in a gladiator pit for their survival/release from prison, and I would like to design/modify an Orc to be the challenger.

Obviously a CR 5 monster would be way too tough, but I am not sure if the CR system can help figure out what a hard/deadly challenge would be for a single character to handle.

Is there an official guide that explains how to create/modify a monster for a one-on-one duel?

Answer 1

How to do it RAW

Using the Creating Encounter section from the Dungeon Master's Guide (page 81) a hard encounter for a single 5-th level character has a XP value between 750 and 1,100.

For a single character (or for two) the Party Size adjustment (DMG 83) says to use a modifier of 1.5 on the monsters XP value. Reverse engeneering a bit; our XP budget for the single monster becomes \$\frac{750}{1.5} = 500\$ to \$\frac{1100}{1.5} = 733\$. So, a CR 2 (450 XP) should be medium, CR 3 (700 XP) should be hard, and CR 4 (1,100 XP) is gonna be pretty deadly.

This as good a baseline as the books — and probably anyone else — are going to be able to give you. You now have to balance how hard you want the combat to be, how much you are willing to risk the character losing, how much you are willing to fudge hits/hitpoints, how optimized the character is, whether there are any circumstantial effects that apply, whether the character has appropriate equipment (greataxe or club?), do they have spells or abilities that would trivialize certain encounters (flight, etc.), and probably a few other things (how much does your player's dice like them? :p).

If you don't know the power level of the character (you can generally get a feel for this if you've DM'ed for it for a while) and you would like the character to reliably survive the experience, I would suggest going safe (CR 2 area), and would remind you that especially when dealing with few characters, variance is your worst enemy.

Answer 2

This question and answer goes into more detail about how Challenge Rating, XP values, and the party size can interact when making encounters.

As I don't have my books in front of me, I can't give specifics in terms of page numbers, but the Challenge Rating (or CR) of a creature is based on the idea of it fighting a standard sized group of players, namely 4-5 players, of that particular level.

So, for a 1v1 fight, I would suggest trying to find a monster that has a CR of 1/4 or 1/5 the level of the player you wish it to fight. So, for your example, you could have a 5th level character fight something in the CR 1 range, such as a Ghoul or Dire wolf.

If you wish for an easier fight, you could pick something in the CR 1/2 range, such as a Gnoll or Black Bear, or for a more difficult fight, something in the CR 2 range, such as a Gargoyle or Giant Boar. Just be aware of how the creature may match up against the Player, as a tougher encounter with a meatier monster may match accidentally kill a squishier spellcaster, while being a better match-up for a beefy barbarian.