Mana prism

Burn unwanted 3* CE's and Servants
Daily quest clearing
Farm silver exp cards and burn them

Most efficient is event shop/rewards that have mp's

I would farm each daily quest once to get the mana prism reward. The remaining AP of your day can be used to farm the 30 AP daily quest "Ember gathering". It gives you around 7 silver xp cards with every run that can be burned for 7 additional mana prisms.

The occasional gold cards you get can be used for leveling your servants.

With that method you can amass 30 mana prisms per day from rewards plus ~7 from every extra run of the 30 AP quest.

To be more efficient you can also skip the 10 AP + 20 AP quests (3 mana prisms in total) and instead do another run of the 30 AP Ember gathering.
The 10 AP and 20 AP Ember Gathering and Treasure Vault quests are useless in terms of rewards if you can efficiently beat the higher level versions. The low tier Training Ground quests are still useful IMO because you can farm often needed blue and red gems at the same time.