To get Easy Mastery (5 mastery points per card) .. The trick is to attack high level accounts with low defense (points or cards) and win. These are the accounts i have been using to date to farm mastery points.
Note for new players : A fully mastered card provides stat bonuses and carries the mastery points (and the normal fusion attack & defence bonus) upon fusion. So spend some time attacking these accounts to master your deck early on!
you wont regret the mastery bonus you cards will provide!
Rule of thumb -> Level higher then you, you win = more mastery. higher level => more mastery. Max mastery points (as i know 5)