Can not kill boss of the Core Map,does not take damage

As the title states, the boss of the Core Map takes no damage, after activating the two hearts and moving on to the third, the boss did not activate the third heart, instead just stood next to it whilst doing nothing. He became immune to damage and thus impossible to kill, preventing me from completing the map.
The minions kept spawning and were killable and were not immune to damage as the boss itself was.