5e PCs are not designed to have a similar power budget in a single encounter? So trying to make monsters that are "equal" to a PC in a single encounter is like trying to find an animal that has the same weight as an airplane, when there is everything from jumbo jets to tiny general aviation craft.
If you want a monster that, if you fought 8 of them while taking 2 short rests, is supposed to be equal to a PC, it is a monster whose CR is roughly 1/2 of your level.
So for a level 6 PC, a CR 3 monster is very roughly a monster you could fight 3 in a row, take a short rest, fight 3 in a row, take a short rest, then fight 2 in a row. Your chance of winning in the end would probably be under 50-50.
Veteran: 17 AC, 58 HP, 20 damage output per round in melee at +5 to hit.
Level 6 BM PAM Fighter: 20 AC, 28 damage output per round in melee at +7 to hit, 52 HP, 11.5 second wind, 19.5 action surge. SD are worth at least 40 damage (at 100% accuracy) per SR, 45 short rest healing from HD.
Total Fighter HP budget: 52 + 11.5*3 + 45 = 131.5
Veteran DPR: 6
Veteran HP per short rest: 174 first 2, 116 last one.
Fighter DPR: 15.4
Fighter extra SR damage AS: 10.7, BM: 40
174 - 40 - 10.7 = 123.3, divided by 15.5 is 8 rounds of Veteran damage.
Fighter wins initiative half the time, shaving off 1.5 rounds of Veteran damage (from Vets who die before making the attack that round), leaving 6.5 * 6 = 39 damage taken in each of the first 2 SR.
After 2nd rest, it takes ~5 rounds, with initiative victories dropping it to 4. Another 24 damage.
So the Fighter takes 102 damage over this day of duels, and has 131.5 total budget; ends the day with a bit under 30 HP left.
This is a very close day and a relatively optimized fighter (without magic items). But the danger is roughly what I expected.
I also neglected crits, which would help the veteran a bit more.
For non-Fighters, you'll get more daily power budget and less sustained, and often less durability. For some classes (like Rogue), a duel is going to make them usually non-functional (as 5e PCs are designed around working in a party).
But the above appears to be how 5e PCs are balanced; a sequence of fights over a day against a foe. But in a group setting, not solo.
...
If your definition of "even" is something different, I suspect you will have to rework PCs.
When you make it all occur in one fight, piles of the Fighter's abilities stop being nearly as useful. Almost 1/3 of the above Fighter's offence came from SR refresh abilities, and the majority of its durability.
Meanwhile, if you did the same with a Wizard, almost all of its offence could be dumped in one fight.
A level 6 Wizards at-will damage output is roughly 11 at +7 to hit (6.6 DPR on the above Veteran). It has 19 total spell levels, of which it recovers 3 once from a short rest, on top of that.
In a single fight, the Wizard's main is rounds is stays alive to dump high level spells. Each fireball is 28 damage (save for half), and they can dump 3 in a row, before subclass abilities and any optimization!