I got my Achilles 1 week ago and I daily attack to several islands. However I am still at level 13 and he gains very small amount of experience. I had the same number of war with cadmus and I had him at level 17 already. I guess there is bug with leveling of Achilles. Some people from my Alliance also complained about this bug. So is it really bug or not?
This is not a bug.
Each Hero requires a different amount of XP points in order to level up.
Cadmus requiring less than Achilles, it is therefore logical that it takes longer for Achilles to level up.