The leveling system of this game can be a little tricky at first time but it's not so complicated, i'll try to help you explaining this :
There are different ranks and each rank have 10 stage , each challenge completed give you one stage .
Once you are on stage 10 of a rank and you complete a challenge, you will be promoted to the stage 1 of the new rank.
Basically , every 10 challenge completed you get a new rank !
You can see which challenges you have to complete from
until you are on the Legend tab. You can also sort them for category (i.e. fighting , driving)
Also, remember that Griffa (the other leveling system for your passive bonus) allow you to get a promotion to the next one stage for ten times !
I hope i was clear if you have any question or suggestion regarding this achievement or a certain challenge i will be glad to help you !