Ask your self this
who do you think is strong enough to fight/scar whitebeard in his prime, whitebeard the only man that was able to fight Rogers as an equal, and come out alive
again it wasn't stated that who ever gave WB the scar made it to him when he was on his prime or if WB is weaker now or if roger had a weapon or a DF or something else to scar WB that i doubt he had
another thing you might not know is that shanks though he lost his arm hasn't became any weaker cuz oda stated so since he could use both hands equally
also mihawk could and i explained in my previous post why
it is clear that some people that think roger is the "bastard" are new here and are confused with fillers and never read the SBS or jumped after an episode from the anime straight to the manga
go read in msn one piece manga v2 the first chapter and the rest and you'll see what i mean about fillers
also dragon appearing in roger's execution and usopp's dad fighting the marine (i doubt he is that weak only cuz usopp is his son) are fillers