LEVEL & SKILLS
There are several way that you can gain new level and unlock new skills. The most common way is to hunt any monsters in Open World for easy EXP. The faster way though, is to win any arenas or campaigns, which you will recieve a lot more EXP at the end of the mission.
You can also use a blood stone to instantly give 200 EXP boost to your demon. This item is rewarded through out the game and can be purchase at the money shop.