not much is known about this character but its clear that he slayed so many dragons that he became one. he is one of the most powerful characters in fairytail this is why he poses as a threat to natsu and all who oppose him.
we know that he slayed many dragons so this is why its a mystery. we also know that he possesed dragon slayer magic. this says that he had no particular type of dragon slayer magic and that he possesed all forms of it .
in order to sleigh a dragon you must have the abilities of a dragon so there is no doubt he used this power to his advantage.
the obious ones like roar or talen or claw and so on
but the others all have their own unique spells.
if this is the case then he got stronger with every dragon he slayed but this is only a theory so if you are creating a character that has these qualities feel free to create