Its not about Kizaru or Akainu. NO ONE can hurt him for the first minutes of the battle, cause no matter what attack, he immedeatly regenreated. Kizaru`s first attack ripped him completely apart but yet he easily regenerated. Does that mean he's as strong as an Admiral ? No! Cause endurance is the key, Marco cannot regenerate infinite amount of times. We saw in the war (check the Manga), that even before the Kairoseki chains were around him, Kizaru's simple Lazer Beams made him bleed and feel A LOT of pain - only with a bit of time and will power he managed to regenrate from that. Another few attacks from Kizaru, and there would be no way that Marco could keep regenerating from that. Marco is extremely powerful and close to Admiral and Yonko, but his DF ability gets miss judged and overrated in terms of regeneration, while his attacking game, his fighting skills, speed and Haki gets underrated. Marco would be considered for the Admiral position like Gion and te Chaton, but ultimately not make the cut cause Ryokugyu and Fujitora are surely more powerful.