The most important is,
avoid gambling on valkyries, unless you want one specific S-rank as your waifu.
The reasoning behind this is that no matter how strong a S-Rank is, she is very likely to stay at S-rank years after you obtained them, while the A-rank valkyries, that can be obtained by farming, can be upgraded all the way to SS or SSS-rank. Even if their abilities aren't that good, their stats will eventually make them surpass natural S-rankers.
The only exception for this rule is that you'll have one guaranted S-rank among your first 33 pulls of the general gacha. After you get her, the general gacha isn't worth your crystals. And even for this, if you're patient, you can save your crystals, you'll get tickets for the general gacha from various sources.
Weapons and stigmatas. That's what your crystals should be used for. Among them, Oath of Judah and the Planck set should be priorized.
I have found this site, that can act as a wiki of sorts.
http://houkai3rd.arthobbylab.com/
The Valkryries and stigmata database is updated to the japanese version of the game (currently 2.5) and good to compare sets and decide where you want to invest your ressources. There's also a weapon database, but it is incomplete, some weapons are missing.
