Well, farming is considered commonplace in all trading card games and Marvel : War of Heroes is no different.
Farming is the repetition of a zone in order to gain some items . In this case the desired resources are cards that can be used to boost a main card and the zone is the specific operation.
The best operations for farming vary based on what type of card your are boosting and can fall around different philosophies.
Philosophy 1: My Philosophy
You should try to use bruisers to boost bruisers, tactics to boost tactics, and speed to boost speed for best results in boosting. Because of this, I suggest using Operation 2 Mission 4 when wanting to boost bruiser cards. This is because 2-4 drops a common speed, common bruiser, and an uncommon tactics. I prefer this level because uncommon has less chance to drop, thus ensuring larger amounts of bruiser/speed cards for enhancement. I choose to farm 2-4 for bruisers only.
I choose 2-3 for farming both speed and tactics because it drops common speed, common tactics, and uncommon bruiser. I save any random uncommon bruisers for future bruiser boosting.
Philosophy 2: The alternative
Since level 2-4 drops uncommon tactics and level 2-3 drops uncommon bruiser some people prefer to farm those for their tactics and bruiser needs. I personally don't like this strategy because although it will net uncommon cards of the same alignment they drop less frequently. But different strokes for different folks.
Quick Farming Tip: One quick tip when farming 1 Energy cost levels. Everybody knows it takes forever but if you progress through the mission until the first reward is given and hit proceed, you can continually click back/proceed to quickly farm the level. The reason I recommend this method is it saves you a couple second each time by removing the need to scroll to the proper mission.
Boost Tip: Uncommons have skills and are easy to farm so if you have high level cards that can be skilled up, save your skilled boosters for those cards. I will note I love the fact that skilled boosters are available at 1 energy cost levels.