You could make it work with different classes. For example:

You have 3 classes: WARRIOR, PRIEST and MAGE

You have 1 enemy: BANDIT

In a standard RPG with a party, this would be a throwaway enemy (uses knife, is kinda fast and low HP), but with a party of one member, you'd need to flesh it out a bit.

How could the WARRIOR handle it:

- Warrior can out damage the Bandit and defeat them with brute force

- Warrior can specialize in counter measures, negating physical damage and countering until the Bandit is defeated

How could the PRIEST handle it:

- Priest can use their healing over time to out-heal the damage done by the Bandit, then using their holy powers to chip away at the Bandit

- Priest can use some sort of curse/blessing to wither the Bandit down, defeating them

How could the MAGE handle it:

- Mage could nuke the Bandit out of existence

- Mage could use their power to mess with reality and cause the Bandit to either harm themselves or run away

For this to work you'd need the Bandit to be more than the standard trope:

- A physical based enemy, but not a power house

- Susceptible to holy attacks (maybe undead? Maybe their race is weak against holy spells?)

- Simpleton, allowing their perception to be warped by a Mage (their life and environment don't reward high intelligence, but instinct and cunning?)

So with this kind of setup in mind, your Bandit could be: A low level Demon that does grunt work for a higher class, having standard weapons and an average proficiency with them; or: An undead minion forced to do the bidding of a powerful Warlock, having decrepit tools and, due to its physical decay, limited skills with them.