Disables + debuffs are your friends here.I am in the Ancient Ruin and the monsters are horrible in there.
One fight has 2 Shambling Terror, 1 Ghostly Terror, 1 Skittering Terror, 1 Creeping Terror.
I die after the 2nd round. I had this issue few times, but I have no idea how to win this fight this time. (Aside opening the RPG Maker and reducing the hit points, so the monsters are killed within the first round...)
Try Vhala's defensive rains to debuff the shambling terrors, Simon's unified strike on either the skittering terror (stun) or the creeping terror (blind), Robin + Qum disrupt on the ghostly terror (confusion).
Other characters could also work, as long as they can do similar strategies.