The fifth level is called the "Difficult to Cultivate" because it involves practices that are so arduous and require a great deal of effort to perfect. It is also called the "Difficult to Overcome" because when one has completed the training of this level one has profound wisdom and insight that are difficult to surpass or undermine.
According to Nagarjuna,
"The fifth is called 'the Extremely Difficult to Overcome' since all evil ones find it extremely hard to conquer him; He becomes skilled in knowing the subtle Meanings of the noble truths and so forth."
Bodhisattvas on this level cultivate the perfection of samadhi. They develop strong powers of meditative stabilization and overcome tendencies toward distraction. They achieve mental one-pointedness and they perfect calm abiding. They also fully penetrate the meanings of the Four Noble Truths and the two truths (conventional truth and ultimate truth) and perceive all phenomena as empty, transient and prone to suffering.