Once you’ve selected the base and booster card, scroll down and click on Boost.
Another way to increase the stats and mastery rating of a card is to use the same card in every battle.You will get a card with increased stats.
In Marvel: War of Heroes, losing a battle may mean your opponent will capture your cards. You too can capture an opponent’s card if you win a card-based battle, but it hurts when someone takes you’re your precious cards. To avoid this, you can protect one of your precious cards from going to an opponent.
To do this, tap on Boost > Card List. Click on the card and then tap on Protect This Card.
Card protection also has other benefits. Not only can you avoid it from being accidentally sold, but also avoid it from getting fused with another.