The fast way to level is leveling up Pfull. Basically you buy a level 4 out of 5 card that is near its max level and level it up. This generally cost about 300-400 extra clintz per card, so it is very expensive.
Well. To really level up ... you need to do urban rivals as complete set. You need to make money in the long run to keep XP flowing, and you need to get new cards so you can unlock new missions for boosts in both cash and cards and credits. Meanwhile you have to play alot, DM will give you good boost, but DT and ELO will generate better profits in the long run - as do survivor.
Meanwhile you can play market, and while doing that you can gain levels by spending surplus XP to level cards. So you kind of level when you play market too, especially if you ever get so rich you actually afford pfulls.
My best advice still is : Dont get bored. Have variety to keep you interested. Try new clans. Have fun. That will in the long run keep you steadily advancing than rushing and getting bored to grinding levels.