My usual go-to strategy is to shield 3 party members and put the remaining one in Safe Spot to conserve as much as possible (I've yet to encounter the 4x Terror Fiend formation, but I'm sure I might end up resorting to something else depending on the situation). If the (Terror) Fiend(s) has/have other monsters accompanying them, I don't immediately excite party members that don't target all with their regular attacks. Once the non-Fiend monsters are defeated, if I feel the damage won't be adequate enough before a new round begins, I'll excite the rest.
For early-game without Shield Sprinkles (or if you're doing a challenge run), put the main physical attacker in Safe Spot until the Fiend(s)' turns are done and hope for the best (though if you're already at a point where Hyper Sprinkles have been unlocked, the luck factor is reduced).