Finding a path towards the brain of the First Born was not difficult, and in reality, the brain area was sure to be near the core of the planet. Once you were digging in the right trajectory, you would not be too far off. With the nervous system mapped after a planet-wide scan, locking on to the brain was a simple task.
Technically speaking, it was easy, but Hao Ren still proceeded with extreme caution.
The First Born planetside was brain dead, but that did not mean that it was not a threat. Those tentacles, the protector of the main body, equipped with means to eradicate any outside threats could operate autonomously even without the control of the brain. They would automatically destroy any threats around it and block all access towards the planet's core. If not for that, the Executor's war to reclaim their homeworld would not have become such a meat grinder.