Are there any important tips for designing manageable encounters for a solo campaign?

[Morvram]

I am currently running a solo campaign; we have played for a couple of hours and so far no combat has ensued but I believe the PC will likely get into combat at the start of the next session. It's against a single opponent and she has a tanky NPC ally in case things go south, but designing this opponent got me wondering if there is any time-tested method for making combat encounters more viable for a single PC without either having the encounter become totally trivial or it being so threatening that some tanky NPC needs to step in and steal the PC's thunder. Do any more-experienced GMs want to weigh in on this?

Edit: Realized I didn't mention this in the original post but the PC in question is a Warlock, so: probably kind of squishy.

[Unoriginal]

A single PC should be able to handle 6-8 encounters of a CR equal to their level divided by 4 (ex: at lvl 1, CR 1/4), with two short rests between them. It doesn't mean you should build encounters like that, but it's an idea of how much this PC can handle.

Groups of enemies and enemies with Legendary/Lair Actions are going to be particularly tough for them, since they have way less actions than their opponents.

IMO I would have them gain some experience by fighting 1 vs 1 combats in an arena or other competition where killing your opponent's not allowed.

It would also help if the NPC and the PC were an unified team rather than the PC seeing the other helping as "stealing their thunder".

[Morvram]

A single PC should be able to handle 6-8 encounters of a CR equal to their level divided by 4 (ex: at lvl 1, CR 1/4), with two short rests between them. It doesn't mean you should build encounters like that, but it's an idea of how much this PC can handle.

Groups of enemies and enemies with Legendary/Lair Actions are going to be particularly tough for them, since they have way less actions than their opponents.

IMO I would have them gain some experience by fighting 1 vs 1 combats in an arena or other competition where killing your opponent's not allowed.

It would also help if the NPC and the PC were an unified team rather than the PC seeing the other helping as "stealing their thunder".

Thank you for the advice!

[Unoriginal]

Thank you for the advice!

You're welcome!

Just to put this in perspective, though: In a fight of 1 lvl 1 PC vs one goblin, who is CR 1/4, the goblin as a decent chance to win. Not a big chance, true, but still enough that it need to be acknowledged.

[Kane0]

What level?

[opaopajr]

More friends, better. However, don't let it go to the player's head. So keep a mental separation between NPC-ally utility.

Remember, there's no such thing as a "perfect slave/servant," not even amongst our world of machines and electronics. So naturally do not give such 'perfect power projection' to a PC. The solo player's life is eased by NPC allies, but it is also complicated by them in turn -- and that is a major part of the fun of allies in general.

The old divide was servants, hirelings, henchmen. The 5e terminology would be more akin to unskilled hirelings, skilled hirelings, and full NPC-ally. (I am loathe to say GMNPC due to the loaded nature of the term, fear of Mary Sues, but it is technically correct.) Here the key to remember is morale, morale, morale. Sapient things can take complex direction; it's their proclivity to obey during mortal danger stress that separates them.

The next major divide is capacity and obedience, often present in what is borrowed from MMO terms: "pets." Animals have limited capacity versus sapients, so that is where you can best complicate them. Naturally morale can just as easily come into play, as animals do default to instinct when stressed.

Summoned spirits (celestial, fey, infernal, etc.) are given stronger capacity in that they are sapient enough to take direction, still in an atypical form, and often tethered by obligations of obedience. This makes them excellent servants when within domain of the PC's watchful eye. But remember, there's no such thing as perfect slave/servants, not even automata.

Complicating summoned spirits and automata is done through their drives and needs. Spirits are driven by their ethos from extra-dimensional power structures, and have sapience enough to find wiggle room around direct orders. It is only a matter of time for a PC to achieve enough domain distance, (or command leeway or conflict,) from this servant that it will go off and 'do what it understands best' or 'was also told to do'. Similar with automata: their drives are more fuel needs and command scripts, which if you've noticed our modern life easily gum up and glitch quite readily over time.

So the answer is: Allies.

And the restraints upon Allies are: Morale, Capacity, Loyalties (plural), Drives, & Needs.

[Cespenar]

Put her in interesting multi-faction situations, like some bandits vs. some peasants, or some mercenaries vs. an ogre or what-have-you.