When you battle an opponent, cards in your attack deck will gain an increase ranging from 1-5 depending on the level of your opponent. If your opponent is lower than you in level, you will either get no increase or +1 increase, opponents with the same level will give you a +3 increase and those above you will give you either a +4 or +5 increase. Take note that this will only happen if you win the battle against an oponent whose modified defense is above 1,000.
Mastery bonus is also given as a random reward while completing missions. Cards that are placed in either your ATK and DEF deck with the highest priority (not including the Suggested Decks) will be picked at random to receive the bonus. If all of the cards in both of these decks are fully mastered, then a random card from your collection will gain the bonus. The mastery bonus received will be equal to the energy requirement for your chosen mission. For example, if you are doing Operation 11 you will gain 10 mastery increase.
The ATK and DEF bonus gained from a mastered card is approximately 37.5% of the base stats. However, this is not an exact number and actual stats may vary.
In order to maximize card strength, a card should be boosted to its maximum level and fully mastered before fusing it to the next rarity. While maximizing mastery provides a direct boost to the cards, during fusion it also contributes a boost to the mastery of the final card equal to its current level.