Arena. It's taxing, but that's the way to go. If you do 1.7 in both featured and basic, that should usually score you 200 shards per arena. Do it twice a week, and that's a total of 800 a week. Also, you'll be getting lots of 4* shards from those, which can get you shards if you dupe. Obviously, if you don't have the roster, focus on one, and then do what you can in the other.
Do quests. Monthly quests are good, and they are increasing the amount you can get. Do full explore of the most difficult one for you, and then fully explore the next one down. This will build your overall team over time.
Get in an active alliance. AW, Summoner Advancement and so forth will help.
Most of this isn't "really" easy, but it's what's necessary. The only really easy ways to get shards are to buy every offer that comes along.
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Do quests. Monthly quests are good, and they are increasing the amount you can get. Do full explore of the most difficult one for you, and then fully explore the next one down. This will build your overall team over time.
Get in an active alliance. AW, Summoner Advancement and so forth will help.
Most of this isn't "really" easy, but it's what's necessary. The only really easy ways to get shards are to buy every offer that comes along.