While playing through the Deadpool Raid Event in Marvel WoH, I noticed a simple but effective way of "farming" your cards' mastery.
Here's what you need to do:
Fix your ATT and DEF decks. Cards within these decks are the candidates for mastery increase. If an unexpected card gets mastered, check the system's "suggested" decks.
Proceed with the latest Raid Event Missions continuously. If you get a Deadpool fight, proceed to step 3.
In the Deadpool fight menu, do the following:
Use Light attack (1RDS);
(Optional)Ask for Support from your team; then
Go back to playing the Raid Event Missions, step 2 above.
Marvel WoH: My Page. Click on "Newest Event Mission"