Most viable way to farm Mana Prisms
i need 3900 mana prism for caldea lunchtime how do i get those while its available.
Aside from participating in events, which frequently provide a few hundred mana prisms, the most efficient method I know of is doing the 30 AP quest for Silver experience cards. Those can be burned in the shop for 1 Mana Prism each, and you'll usually get 7-9 per run. If you have your 3-star servants ascended already, you can also burn extra copies you get from the Friend Point and/or Quartz Gacha for more prisms.
Mana Prisms are most efficiently collected over time. It's hard to rush them without mind-numbing amounts of grinding, so it's better to collect as you go if you can. (This is, of course, a bit inconvenient for those who start the game with something they want nearing its deadline...)