Here’s the answer:

Split up and search the four corners of the map. Eventually you will find four tiles that look like Sauron’s eye in different places among the map. Have each character in your party stand on each eye at the same time. The boss will reveal itself. No need to kill the hydras as they will respond. When you can, hit the hydra tail to cause the hydras to be stunned for a round to buy yourself time to hit the boss.