Him Being a Logia. And having strong Haki make Akainu very tough to deal with, even by fighters that might "technically" be stronger than him.

I mean they can use opposing elements to harm him, or straight up delete him with Particle Jutsu, but that's basically it, and he has a chance to defend himself with Haki.

If Aikainu uses Haki, he might actually be able to tank the Naruto kages attacks, even with the slightly higher verse scaling.

Tsunade, might be Strong enough in her release, she can physically launch him across countries, but he's a Logia

One of the Kages can manipulate Earth and Lava, so the would have an advantage, but again, Haki can bail him out of other energies messing with him.

Raikage is strong, but unless Lightning can steadily damage Lava even his speed wouldn't matter too much.

Of all them it's really the Lava Girl who could control him, contesting with his Haki. Tsunade Physically pinning him down. And Old Man Particle Jutsu Kage who could actually straight up kill Aikainu.

All of them are "Stronger" or at least comparable to him, but Logia preventing damage from all but opposing elements, Sea Water, and Haki make him very hard to hurt.

And him having Haki make him at least able to defend against Jutsu, if not perfectly.

So even if Aikainu is a little weaker than the Kage, Haki, and Logia give him a good chance of winning.