As Abu said, sheer plot power.
The Red Dragon, ok, even if it's already a bit far stretched I can swallow it, he trained for 3 years exterminating alone whole dungeons and trained an hardcore style, while his lvl could be lower than average his skills should be maxed, let's say skills are harder to up than lvl, this is then entirely in the scope of what could happen... for the Red Dragon, that is.
OHK the King Dragon which is most likely at least a field boss and maybe a regional/world boss ? Come on...