I will hit rank 10 chocobo very soon. This has been my secret to leveling one very quickly.

Wait until about lvl 45 to start ranking him up. Travel to Dragonshead. The bottom left corner of the map is an area called the fury's gaze. there are about 10 lvl 48 plasmoids that spawn there and do not aggro. They respawn fairly quickly. Party up with three other people and all summon your chocobos. AoE farm and Level exclusively from 45-48 grinding these enemies. You'll get a 10 chain every time they respawn.

After that, take your party to Ceruleum Processing Plant and run dark devices fate with chocobos out. Dont worry about killing, just tag everything. You'll be about rank 7 chocobo once you hit 50.

Now repeat this on another character. Once you're at 50, he will be close to 10.


p.s. he's still useless at R9 so it may be a waste of time altogether.