Leveling up Servants is essential for completing harder content. Servants gain levels by sacrificing other Servants or Experience Cards to them. This can be performed from Servant Reinforcement, the first option in the Synthesis menu.
As of Version 1.16.0, the maximum number of slots for "Sacrifice" was increased from 5 to 20.
To level up Servants efficiently, simply use the best Experience Cards you can get, and aim to only reinforce with the matching class bonus whenever possible. If you want to level up classes other than the original seven, you may want to save the ALL-type Experience cards since only the original seven classes have their own dedicated class EXP cards.
Every time you feed EXP to servants or craft essences, there's a chance to result in a 大成功 Great Success (2x EXP acquired) or 極大成功 Super Success (3x EXP acquired) in JP server; Super Success (2x EXP acquired) or Great Success (3x EXP acquired) in US server.
Once your Servants' levels become higher, the amount of experience needed to gain a single level becomes steep. At this point, it is best to reinforce using only 4★ EXP cards, and burn 3★ XP cards for Mana Prisms instead.
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As far as farming EXP cards, because of the huge discrepancy in usefulness between 3★ and 4★ EXP cards, it is best to run the 40 AP daily training field quest if possible.