User blog comment:IamJakuhoRaikoben/Chapter 402: Iron Fist of the Fire Dragon, Review./@comment-24546844-20140930003119/@comment-7418318-20140930120837

Didn't the Twin Dragons only say that they cuoldn't leave a mark on him, not that they couldn't hit him? Or am I misremembering here?

If I am not, it might mean that they did manage to hit Mard, but their blows were too weak to bruise or scratch him. All that Natsu did was leaving a small bruise on his cheek, but didn't seem to have done any actual damage. Considering that Natsu (without using LFD Mode) could overpower a Dragon Force enhanced Unison Raid by the Twin Dragons, it's quite natural for his hits to be much stronger than anything they could dish out and this difference in strength is the difference of whether the hits bruises or is just shrugged off by Mard.