They are highly variable in their performance. Sometimes they're fodder that streetlevel heroes can mow through while in some cases their punches can make Superman grunt and they can fight Amazons (beings who also vacillate between enhanced humans with impressive martial skill and Class 100 supers who are all consummate warriors) on even footing.
In most cases I'd say Parademons are like enhanced humans with Deathstroke level strength but little to no real skill and usually shitty damage soak (but a high tolerance for pain at the same time).
__________________
“Where the longleaf pines are whispering
to him who loved them so.
Where the faint murmurs now dwindling
echo o’er tide and shore."
-A Grave Epitaph in Santa Rosa County, Florida; I wish I could remember the man's name.
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In the previous Flash series parademons invade and were semi-repelled by regular police...whose bullets pierced and killed the aforementioned parademons. In Cosmic Odyssey one-on-one they were stronger and more durable than Batman...but Bats shot and killed it with a New God gun.
They just seem like cannon fodder to me. They've never actually slowed down Superman or Orion or Krypto when they've attacked Apokolips, just gotten in their way and brushed aside.
I think there was something actually said about how much power being channeled into them varying... though I don't remember the specifics. Maybe something to do with the amount of despair around or something. Immune to pain and stronger than human is standard, but how much can vary, and in big enough number they're a threat to even fairly powerful heroes.
The Parademon that was in Villains United as one of the early members of the Secret Six was quite strong. Also way more independent than normal, so exceptional across the board, really.
Most of them are taken from the most ruthless of Apokolips's Hunger Dogs/, transformed, then as a primary job are then used to control and oppress said Hunger Dogs and brutally quell any riots or problems.