My biggest tip requires that you play the long game. Each Shards event has a collection of new girls and all of the previous event's new girls at a discount. I opted not to buy any of the new cards from the last event with the exception of the N card because they are always 100 shards.

Supposing that I manage to get all of the previous 'new' girls, I expect a total shard savings of 300-500 shards. This is pretty big since it is entirely possible to get screwed by the RNG gods during your e-gacha spins, particularly if you are free to play.

After you buy all of the girls from the previous event, STOP buying girls. Your patience will be rewarded next even when you can buy them at a discount. Sure, it means always being one event "behind," but it also means that you are more likely to expand your card count and SED overall. It only requires patience and will power.

The catch is, you have to buy them or lose them forever, but if you are serious enough to use my strategy, you are probably serious enough to get 300-500 wins in the event and get at least enough slayer cards for a bonus of +4 or higher. That is more than enough to get all of the previous girls.