Each BOSS has a certain probability of releasing a [Basic Skill Book] after being knocked down, and the skill books dropped by the BOSS in each level are different. Normally, when a card is placed once in battle, the skill proficiency of the card will be +1. When the proficiency reaches the upper limit, the skill level will be upgraded by one level. For example, after the Double-layered Xiaolongbao skill is strengthened, the attack speed becomes stronger. The more you use it, the faster the attack speed becomes.