Is there any marine who could 1v1 a hive tyrant? Not including plot armor like the named Blueberries have. My guess is the answer to this is a hard “1V1 would just be canned dining for the tyrant” but I had an argument with someone who thought otherwise and would like for you to opine.
Semi related but how different is the Swarmlord to a regular tyrant?
Are there regular marines who could solo a Hive Tyrant? Absolutely not. Just no. They'd be so dead. So quickly.
Are there specific named marines that could fight one? Yeah. Ahriman, Kharn and Typhus definitely could, though it would still be a hell of a fight. Dante, Moloc, Calgar and Tyberos probably could, too. These are some of the most individually exceptional astartes we have in the setting, mind you, and they'd all still come out of it looking fucked up.
The simple reality of it is that you don't want to throw hands, by yourself, against a ~20ft tall, intelligent, hyper fast/strong psyker with a bunch of limbs.
As for the Swarmlord, the Swarmlord is basically a modified, physically elevated Hive Tyrant that's been implanted with the recurring, expanding collection of memories and behaviours that make up the Swarmlord's "mind". It's a Hive Tyrant with even more exceptional physicality, and a vast and nuanced understanding of combat, tactics, species, fighting styles, etc etc. It is old and it understands how to react to and work around an ungodly amount of data.
When people get irked at the Swamlord not killing Dante or Calgar or whoever, this is why. The thing is, by all lore accounts, the physical and intellectual equivalent of a Tyranid primarch. It is so fast and so strong and so clever that, on paper it should readily tear most characters apart. AND it gets better at fighting and waging war after each encounter.
Much like the Avatar of Khaine, what it is versus what GW allows it to do are two very different things.