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?