While the Rock and Cena are obviously ridiculously strong, don't sleep on Seth.
Cards can be won from the power up dungeon battle - there are on rotation every week to give different ones at different drop rates. Material cards can be won from various quests. Action cards, which unlock the ultimate move for your character, are rare to get but can be found from the power up dungeon card battle. More often, they are dropped when summoning for a FES character’s action card. In the exchange shop, 4 special Action Cards can be exchanged for one. They will not exchange why cards that are equipped. If you need a particular option card, 6 option cards of 5 star grade can be exchanged for one. Most of the 5 star cards are useless. Don’t waste time with cards that will give you a 2% increase in an ailment damage- remember to check the codex. While everyone will tell you to get the Kula set, some sets will actually give you a higher CP for your strikers such as the Chizuru or Billy Kane set.