The fight with the colossal fiend that is suppose to banish one of your units and then summon a phantasm is bugged.

The first time the fiend banished a unit of mine, it actually worked. After I had the map trapped with pitfall trap (+) however, it stopped working properly. Each time the boss tried to summon a phantasm, it was instantly killed and non of my units were banished. The boss simply refuses to banish one of my units if the phantasm instantly dies.