I've always assumed those past slayers have Dragon Force, so that was what gave them the edge. It remains to be seen in 2G slayers have the force. I do believe it's possible, given they have other dragon traits (lungs, smell, hearing, motion-sickness etc).Well in the past Dragon Slayers killed Dragons.I believe they should be on the level of God Serena.
I'm not dismissing that Serena can defeat a dragon with his spells, just not sure if he can stick around long enough to win with his durability.