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I'm in the process of creating a campaign and in the time leading up to it I've been running a few duets, or one-on-one, campaigns via a play-by-post format. I've been struggling though to create tense or threatening combat without being outright unfair. I'm used to creating combat encounters in which there is a full and diverse party, a couple casters, martials, and utility, but balancing combat for a single player is quite difficult, especially when one of my duets is with a Cleric, and another is with a Rogue.

The players are able to temporarily recruit companions during the Duet, which will usually just be NPCs or friendly creatures, using CR rather than essentially giving the player a second character (entering Trio territory). They will only ever be able to have one of these companions but will typically be alone, when the companion is with them, they won't typically provide active support in combat, and are usually with the player to be protected by them rather than the other way around.

I'm confident with creating encounters for a lone Fighter, Monk, Barbarian or other frontline class, but less so when it comes to pure casters and utility classes like Clerics, Rogues, Bards, etc.

If it helps, I am running a homebrew module and setting, so I have large amounts of flexibility when it comes to how I run the encounters. I also have access to most the official sources for creatures and will use a variety of them, including re-flavours to mimic combat diversity.

How do I balance the combat fairly in a duet campaign for non-frontline classes without diminishing threat or perceived threat?

The default D&D combat flow is not intended to do what you are trying to do; it works really well when pitting two roughly equal groups of three-to-six hostiles against each other, but too many or too few on either side lead to drawn out or static encounters respectively. In encounters I've run where a PC is scouting alone, I've found that it really runs best when not run any different than any non-combat encounter, save for the fact that you're using weapon proficiencies and armor classes instead of skill proficiencies and difficulty classes. This also frees you to run a scene that is more dynamic than a straight up murderfest. As far as what kind of enemies, the default assumption of D&D is that the PC's are supposed to win, so make sure that they're weak individually (CR1/8 or CR0 for the levels you're talking about), but you can make them plentiful. This frees up strategies for the lone PC to pick them off, one by one, Batman style, and use distractions to clear the way. Depending on what kinds of enemies you're facing and the story you're trying to tell, bribery and deception could also be tools the player could use.

Example similar to a game I ran:

The Setting: The PC, a rogue, wants to get into a guarded room because he thinks the McGuffin is in there. A guard stands beside the door in a hallway. The rogue is standing back assessing the situation, unnoticed by the guard.

Rogue (the PC): I want to sneak up on the guard.

DM (me): The hallway is mostly empty; he'll see you coming.

Rogue: &$%@. It's a hallway, right? Where are the other entrances?

DM: (draws a quick sketch of the encounter area.) Here's where you're at, here's the guard and the door, here's a door that probably leads back to the dinner party you just came from, and here's a door you don't know where it leads. There's some paintings decorating the walls and a vase on a display table here.

Rogue: Okay, I take one of my daggers, and toss it into that other door.

DM: The dagger makes a loud noise as it bounces off the walls of the unseen room and skitters across the floor. The guard jerks upright, then walks toward the door. He casts a glance back at the door he was guarding, then steps through the doorway the noise came from to investigate.

Rogue: Yes! I try to walk quickly, but quietly, over to the door [the one with the McGuffin behind it].

DM: Alright, give me a stealth check to do so quietly. DC is 10.

Rogue: Ouch. I rolled a 2, after my bonus that's only an 8.

DM: It's no good. You boot scuffs the floor as you walk past the door, making a noise. You see the guard start to turn. You have a moment to react.

Rogue: I whack him in the head with the flat of my shortsword!

DM: Give me an attack roll, with a +2 bonus since he's not expecting you.

Rogue: 18. You said he's not expecting me, does that mean I can roll sneak attack damage as well?

DM: Yes, please do.

Rogue: Eeeeeh. Total 8.

DM: You rang his head like a bell, but now he's mad and in striking range. He snatches his club from his belt and takes a swing at you, roaring like a bear. Does a 17 beat your AC? He deals . . . 4 bludgeoning damage if it does.

Rogue: It does hit. Well this went as bad as it could. That first attack really hurt him, right?

DM: Yup, he's still a little cross-eyed.

Rogue: Alright. I can't leave him to identify me, so I try to stab him between the ribs.

DM: Attack and damage, without the sneak attack bonus this time.

Rogue: Ha! Critical hit. 13 damage.

DM: You killed him. What do you do now?

Rogue: Somebody probably heard that. I'm heading back to the dinner party.

In summary: Use individually weak enemies; leave props, options, and landscape open to allow improvising players to set the fights up to favor them or to avoid them altogether; put decision points at places that will allow them to avoid combat if they so choose; and don't be afraid to set combat rules aside for the sake of a more narrative approach.

Preparation is key.

The player's job is to assume mastery over their abilities and understand how to mitigate the dangers of an encounter. Single combatants have the sole responsibility of this since they have no other party members to rectify mistakes, and a random critical hit could mark their end.

Different classes each have their own unique ways to prepare for a known encounter.

For a level one or two cleric, this would include preparatory spell-casting such as Shield of Faith to raise their AC by 2, lowering the chance to be hit. If there are multiple enemies, Command could be used to temporarily neutralize one by making them flee or fall prone (or move in a way that is not beneficial, like having a ranged spell caster approach you).

For a level one rogue, this would be identifying environmental sources to generate advantages on the attack to utilize sneak attack. Once the rogue is level two, environmental sources of advantage are not required since they can use Cunning Action to hide. Another mitigation technique would be for the rogue to identify if there's a way to avoid the encounter entirely, since they have no class-specific way of recovering HP.

As the GM, you balance encounters by highlighting important factors to players such as the enemies capable of long-distance attacks (perception to notice the bow on their back), sources that could generate advantages or disadvantages for the enemy (bonfire is their only source of light, the flue of the chimney can be shut to fill the room with smoke), or even enemies that are out of their league and they should avoid (insight or nature to know a single knoll is would be a pretty tough fight and they shouldn't engage the pack of three). Rolls should be encouraged to create logical advantages, disadvantages, or distractions.

Combat is deadly.

Encounters with only a single PC combatant is very dangerous for the PC. If the PC falls to zero HP and goes unconscious, they are at the whim of the GM for what happens next. Unless you want to have this contingency built into your plot line, then you must emphasize the dangers of combat to the player.

A Cleric at level 1 has 13 HP max. A bandit deals 1d6 + 1 damage on hit. This means no matter what preparation the Cleric had, there's a small chance they die in a single strike, though at 1/720 odds it is unlikely. The bandit's average of 4.5 HP means it'll take 3 hits to kill the Cleric.

I can't do all the number crunching for the whole breakdown of how many turns the combat with a single bandit would last (others can do that far better than myself), but needless to say the Cleric will likely have to rest before a second encounter.

Not all combat is necessary.

Because combat takes such a tax on a single combatant, it may behoove you to include ways for combat to be avoided entirely that observant or clever characters can discover.

Stealthily setting fire to a bandit's tent could distract him long enough for the Rogue to free the farmer's daughter and abscond with her.

A Cleric admonishing the bandit and appealing on behalf of the bandit's eternal soul could intimidate or persuade the bandit to turn himself over to authorities.

Sometimes, both combatants realizing that death is on the line could, in and of itself, be the reason why neither wants to fight and lend itself to a conversation instead.

Ultimately have fun.

If death/capture in the first few levels would endanger the campaign, build in a way to guarantee the characters make it to level three before death is a real danger.

You could also change the framework so that the first two levels are being told through flashback, so that if a character mechanically fails, you can then narratively recover by explaining something that brings them closer to the present such as a mentor that helps but sticks to the shadows until needed, a group of travelers happening upon the unconscious body, or have being captured be the way the two now meet up (rescuing the other character from a bandit or goblin pack, for instance).