This has two parts: (1) the etymology of the name of the fifth ground and (2) becoming superior in meditative absorption and mastering the truths.
The etymology of the name of the fifth ground
This great one on the ground of Hard to Conquer
cannot be defeated even by the entire host of māras.
The great one abiding on the fifth ground of Hard to Conquer, if he cannot be defeated even by the entire host of māras such as Devaputra residing throughout the world systems, what need is there to speak of others such as the servants of Māra under his command?
Seeing as the bodhisattva had previously become superior in the first four perfections — generosity up to diligence — it is easily understood that the point of reference here must come from among the remaining six perfections.
This is to say, the bodhisattva has attained on this ground a level of absorption that can never be trodden upon by such opposing forces as distraction, and this extent of mastery is lacking for the perfection of wisdom and the rest.
Not only does the bodhisattva become superior in meditative absorption, he also gains mastery in refined understanding of the nature or essence of the truths of the wise — namely, the āryas — understanding that requires comprehension through refined intellect.
Therefore here he becomes endowed with superior wisdom versed in the coarse and subtle aspects of the truths.
The Ten Grounds Sutra says that the fifth-ground bodhisattva comes to be versed in the four noble truths of suffering,its origin, cessation, and path, and separately it also says that he comes to be versed in the conventional and ultimate truths.
This raises the question of how there can be four truths distinct from the two truths when it is clearly stated in the Meeting of Father and Son Sutra as well as in Nāgārjuna’s Fundamental Wisdom that the number of truths is exhausted within the division of two truths.
To respond to this question, Candrakīrti’s Commentary confirms that there is no truth not encompassed within the two truths.
The four truths, he explains, were taught to demonstrate that what is to be relinquished (the class of afflicted phenomena) includes their origin (the cause) and suffering (the effect), and what is to be affirmed (the class of enlightened phenomena) includes the path (the cause) and cessation (the effect).
Of these, the Commentary states that truths of suffering, its origin, and the path belong to the conventional truth, while the truth of cessation belongs to the ultimate truth.
In his Commentary to Sixty Stanzas of Reasoning too, Candrakīrti explains nirvana to be the ultimate truth and the other three truths [out of the four] as conventional truths, and nirvana here refers to the truth of cessation.