Well the reason Akainu didn't take much damage from Marco and neither did Kizaru from Marcos kick is because even without their DF, the admirals are REALLY strong, and have great endurance. And also they have armament haki as well, so the can use their DF to protect their body, but if someone uses Haki to hit them, they can use haki to protect themselves from taking too much damage.
Whitebeard hit Akainu many times using haki, and you could see that he took alot of damage, but he was still able to fight and still able to hold his own against almost all the whitebeard commanders. That shows the endurance and battle prowess of an admiral
