What is the most efficient way to level up your servants?

Usually do em one at a time. Sometimes I'll do a second one along with if the daily has the second one's class tokens in it. Unfortunately the most efficient way outside of the 100 gold tokens we get from events is still the dailies.

Daily farming is easy though even if it is expensive AP wise and thus takes awhile. Turn one Raikou support NP after waver buffs. Turn 2 give raikou crit buff swap waver for herc, use raikou's first skill and kill each hand with a crit. Turn 3 Fran np. An alternative strat could be to have Fran go first, swap Fran for Arash or some other aoer with a kaleidoscope and have that servant np in between.

Of course...the harder part of leveling up servants is their ascension mats ;) that way Kuro is a good target to max ascend before attempting to ascend anyone else.

To level up servants just farm the 40ap ember quest and get all the xp cards from the mana prism shop as well as the event shop

Event exp card packs and dailies are usually the best way. You can also buy 50 gold exp cards from the shop for 200 mana prisms once a month.

You should focus on one at a time like Neil said. You can do it one of two ways: 1.) Level each servant to their max level or 2) set a high level to reach like 80 before capping out.

The first option ensures that each servant will be at their best while the second option ensures that you have an overall solid team all around the same level.

By the way, since you have her, save up whatever mats you can to max out Squirtle. Even with the fact that you'd have a max level Kuro in the meantime, Squirtle is just that good a unit and will continue to be good even 2 years into the future. She does need A LOT of mats but she is worth it.