Mihawk wanted to find out if WB was still the WSM... He couldnt find that out because this is a war; expect interferences in battle(Jozu block Mihawk attack directed for WB). Now it wasnt stated Mihawk used his strongest move or anything close to it. He was there to "test" WB, then changed course because of Luffy(wanted to test him). Was Mihawk weaker than Luffy? Obviously not, so when we reference Marineford to down play Mihawk(strongest in the verse not name Kaido), because he fought Vista another master swordsmen and everyone else that wanted to protect Luffy, understand thats not a fair setting to determine any one potential. Its war, not an tournament where can actually find out whos the strongest. So Mihawk over Roger Hes the only one alive that figured out how to turn his supreme blade, black while Prime Beard couldnt or Roger (who was weaker than WB).