
At first, it will be hiding among its brethern but you can spot it easily if you use the lock-on and see if no health bar pops up above its head, which means you've found the boss.
But as you can see, once you've taken down its health to certain level, he'll suddenly grow and get far more dangerous with its attacks.
It doesn't help either that it will spawn in more of its brethern if you kill them, so focus on the boss instead and avoid the attacks.