I'm surprised Tanaka's dragon slayer role is still so low after all that.
Also WHAT THE HELL SUKE!? What are the stats of your average high ranking adventurer if Suke can do all that with lower double digit stats!?
Maybe theyre getting some kind of ongoing buff from being in Tanaka's party which makes them 100 times stronger or something?
As for the dragons, I would imagine that the level progress in dragon slayer scales with the power of the dragon, so the first one Tanaka killed was a big deal, but these ones are all brainwashed and were weak enough to be captured in the first place, so maybe they contribute less to the skill...