Although that whole situation with Alissa messed up with his rhythm, Ryan still didn't forget to think about what he learned when he tamed the salamander, so he considered a few possibilities. It looked a bit unrealistic, but it was highly probable that bosses had a certain level of immortality. Although he defeated the elder salamander a few days ago, the next boss had the exact same strength. If he were living a game, Ryan would think that would make sense, but he wasn't. So that was the only explanation.
"Monsters can grow stronger the more mana Earth absorbs… that would explain why we still can defeat certain monsters despite the fact that they probably had decades to grow stronger, and we humans only had a single month. I can understand why the elder salamander was supposed to be one of the strongest monsters in its own world, but I can't imagine Bolvath being the strongest in the world of the demons."
