That would make Akainu ridiculously OP to the point that no one can touch him. I'm pretty a haki infused punch will get Akainu and it'll hurt him.
So far the only Hax DF power is Blackbeard's. How can anyone counter that? That's why I think BB is the endgame boss in OP.
wait..you asked about an attack from akainu to someone else,not someone attacking akainu
haki makes you attack "the user real body",so yes,if luffy throws a punch to akainu,the part that comes in cotnact with his fist would turn back into flash and bones and thus hurt akainu...
but that doesn't mean that if akainu dumps a bunch of magma on luffy,he is gonna ruvive it thanks to haki,since defensively speaking,all it does it's make you harder
i never found blackbeard that OP..actually i thought that in a fight ,if we count before marineford..luffy would have had a fair chance to beat him,mainly because luffy it's strong thanks to his training,not his fruit..even if Teach grabs him,he is still at punching distance from guy able to split two houses from a still position,that without counting the fact that BB takes double damage from attacks (or at least feels 2 times the pain,if that translates to damage too has never been fully explained),puts him at a bit of a predicament.
Also Teach is slow...as in "never dodged a single attack in the entire series" kind of slow...so he would have needed to catch luffy first..and that was another problem for him given how fast is luffy at g2 speed
of course with the acquisition of the gura gura things change quite a bit...now not only teach has obtain a pretty good power up in terms of pure raw power (something he had to begin with),but also his attacks have now immense range and so he can technically catch opponents more fast than him just by spamming gura-fists