This will be an expanding blog of play strategy. All comments are welcome as I definitely don't have all of the answers. I'm probably behind the power curve a little bit, but my goal is always to be competitive... not necessarily top rank. Top rank will always be reserved for those that spend the most money. Freemium games are like that.
MENU --> TEAM MEMBER
Under each team member there is an option to rally them. DO IT! All of them... every day. This is one of the few things in this game that seems to be designed to be extremely convenient. Once you rally everybody on the first page, the game defaults to update to the next page until you've made your way through all of them. I'm at the point that I'm getting 3 free Rally packs every day. I know the cards aren't great... but they're free. They don't even cost XP. Quite simply, it's the cheapest way to farm cards, whether you need them for boosting or for silver.
MISSION --> Scroll to the bottom of the screen --> OPERATIONS --> Operation 2
Mission 2-3 is currently my favorite since I'm boosting Speed and Tactics cards. If I need to boost some bruisers, I choose 2-4 instead. I sell the uncommon drops since I need the 990 silver. However, I will also farm the uncommons to boost the ability level of fully evolved cards. The math is pretty simple... every 3 XP yields 1 card, 800 silver, a cloak, or 1 mastery level.
Strengthen your Alliance
MY PAGE --> [Alliance Name] --> DONATE --> DONATE RESOURCES
Farming Operation 2 will give you mass quantities of capes. I donate all of them to my alliance. I'm at almost 300k donations so far and I haven't shared a single silver. These donations build your alliance, and hopefully your generosity will be rewarded with a leader bonus.
MENU --> DECKLIST
Make an attack deck full of cards that you are trying to max mastery for. Then make it your highest priority deck. This is inconvenient if you battle a lot for resources, but the gain is that you will gain mastery bonus for these cards while working on (or farming) missions.
BATTLE --> SELECT FROM RIVALS
At some point you will find a level 30 with a DEF of 40. My "Mastery" deck is anchored with a ATK powerhouse for just this occasion. Attack them. Attack them again. Do it a third time. Wait until tomorrow and smack them around some more.
BOOST --> RARITY --> LOW
Sort your cards so that you use commons for the low levels. At some point, it makes sense to fuse your common cards so that you get more boost for the silver spent. The boost page lists the silver cost per level. Above level 30 you will save a lot of silver by fusing your commons (for 1575 silver) before using them to boost.