At the absolute beginning of the game, never "eat" your silver units. Eating refers to the act of sacrificing one of your units so your other units can gain EXP, as shown in the second menu. Many of them are actually quite viable. If you have to eat them, at least use fairies and pump up their level to ~21 or so because higher level units yield more experience.
Fairies give 2x bonus corresponding to the proper fairy rarity. Resulting in 8x in total (3 fairies) along with your sacrificed unit.
Cardinal rule of the game: raise who you like. There's plenty of hilarious builds out there (all dragon soldiers, all bandits, you get the idea). Draa: not that they will always work, but favorite > efficiency works most of the time. Since character grows significantly due to affection points and outside of a few character, there are no bad characters to invest in.
Take things slow. Do not, under any circumstance, feed on Valerie (Silver Mage) or Calliope (Silver Witch). This results in instant regret and isn't recoverable for non-cash users.
Valkyries and bandits are best sources of EXP food because the former gives more EXP/per level AND only promotes other Valkyries. Bandits has the second lowest number of useful promotions, hence why they're appropriate. Note, all male characters gives more exp than female. (Silver 150 versus 300, gold 250 versus 750 exp)
If possible, save at least one copy of each silver unit for class changes. CC refers to class change. Think of Fire Emblem - in FE you can class change at 10 but you lose out on stats. Same thing here. Characters CC at level 30, but can go up to level 50. Rule of thumb is to 50CC anything silver+. Don't go for a 50CC on silver unless you're a completionist. There are perks to having an early CC if it means you can get through an event vs. none. Event characters in this game never disappoints.
For silvers, if you don't have any characters you actually like, go for CC'd healer, heavy armor, and mage as your first priorities.
In more details:
•If the silver healer Arisa (long blue hair) isn’t your favorite, go ahead and 30CC her, the other silver healer doruka (short black hair) has more copies available to cost reduce her
•Rean the silver heavy armor should always be 50CCed unless you have a better HA
•Mage wise, unless you really need a warlock, valerie should be 50CCed. Keep in mind most of the time, you can use bronze mage to do Valerie’s job. But some times, a bronze mage’s damage/range just doesn’t cut it
Drops are pre-determined based on when you load in. Mobs will drop the same item consistently - eg. in aerial battle the last mob will always drop a fairy if you "rolled" a fairy for the stage. The 8th mob's always a gem, etc..