The Chaos marine was expecting to crush Cain like swatting a fly, attacking arrogantly with his huge weapon.

Cain manages to deflect and dodge two attacks before the marine gets cored like an apple by Cain's plasma wielding buddy.

Cain did not beat a Khornate berserker marine in hand to hand combat. Cain endured two attacks, long enough for help to come.

Having said all of that, Cain is the protagonist so ofc his plot armour is THICC. But I don't mind this excerpt at all tbh,  He's using the marine's arrogance against himself and then 'cheats' by having his buddy step in with devastating firepower. That's a far cry from, say, Cain fighting the Chaos marine for a full minute and then slamming his chainsword in an armour gap and actually, single-handedly killing the berserker. That would be preposterous.

Below is a book extract.

"The acitic light of Jurgen's melta stabbed the darkness, vaporising the middle of his chest, and he stumled, dopping slowly to his knees."

Cain even said "Not that I had any serious hope of besting him in a prolonged fight".