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 , you have to complete 10 challenge to reache the new rank and get the cheevo!
You can see which challenges you have to complete from Start Button -> LB Button to Legend tab. You can also sort them for category (i.e. fighting , driving)