I do not know if the OP ever DMed a PnP game before. A DM that makes the wise mobs go for the healers is a bad one. A successful DM is one which gives the players a good chance at succeeding but have it challenging. If every boss fight is made impossible by having the boss target the healers repetitively, the chance of success is going to be low. This is why monster manual also includes suggested monster strategies and tactics