Same CP battle

Well firstly I want to point out if it's same CP, let's say 1k, then Parasect is > lvl 20, and Dragonite is lvl 10. A fully maxed 100% Parasect at lvl 40 is 1750 CP, which would only be lvl 17.5 for 100% Dragonite.

That aside, Parasect's typing would put it at a disadvantage. All of its quick moves would be doing 80% dmg (and they're weak already). The only charge move which would do 100% dmg is Cross Poison.

If you dodge like a champion and Dragonite has a bad defensive moveset, like Dragon Breath/Dragon Claw then maybe...with a lot of luck. But probably not.

Any Pokemons at the same CP, it comes down to the distribution of stats, and attacks (effectiveness and strength). Parasect's highest stat being defense and worst being stamina will always hold it back. Plus it has weak charge moves that aren't super effective vs. much except grass or psychic.

Is seems you are wanting to over simplify things in order to make sense of it better. Unfortunately, it isn't quite that easy. There is no "supposed to win" just based off of CP value.

CP value is nothing more than a rough estimate of strength based only on Pokemon attack, defense, and health. But it tells nothing about Pokemon element type or move set, and can't factor in user skill.

IN GENERAL someone who is attacking a similar CP defender, and dodging correctly should almost always be able to win. However, depending on element types, and move sets that might not always be the case. The attacker may only be doing 64% damage, and the defender may be doing 156% damage.

But conversely, if you choose your match ups well, I've had my ~1200 CP units easily beat ~2000CP units.

TL;DR It's not that simple. I'd EXPECT the Parasect to be able to easily win. But without knowing what moves each of the units has, and their IV's, it is impossible to do the math and know for certain.