The Kranos was the personal weapon of a High Ranker who took part in the Great Journey with the FH's and Jahad. You can't base it's strength of it's current owner because you don't like Yuri having an S rank weapon. That's like downplaying the Thorn's because Bam is using them.

Another thing to note is that the only reason Karaka survived that is because he's a defenively oriented High Ranker who leveraged his arms inventory and left arm as a shield. For example, someone like Elpathion isn't surviving a finger blast from Urek like Karaka had.

This is what Yuri commented when Karaka tries to leverage his arms:

"Against an attack that not even High Rankers can withstand?

Another important thing to notice is that one of 'Zahard's 10 Deadly Weapons' El Robina, Zahard's prized Anti-High Ranker weapon, scales comparable to Kranos. Ha Jinsung surviving because of his strength while any other Ranker or weaker High-Ranker would have turned into dust (sound familiar?).

The implication here is that if Zahard's Anti-High Ranker one shot weapons like El Robina number only 10 in the entire tower- then Kranos must be an incredibly rare and renown weapon in the entire tower as well.