It seems to me that rather than providing methods, certain Chan masters who try to prove full enlightment in the span of a single lifetime, engage in a philosphical feints to support their conception, in general resorting to arguments by means of ultimate truth to try and prove their point, basically arguing the doctrine of paths and stages is an unnecessary conceptual limitation. However, when challenged, I don't see a coherent defense being mounted, which has lead me to believe that much like pure land buddhism, Chan is in fact a faith oriented school.
Every Mahāyāna tradition is faith based. One of the glaring shortcomings of Tibetan Buddhism is in taking a faith based, visionary tradition, and trying to interpret it literally. Of course, this began in India, with proofs of omniscience and so on. But that doesn't legitimize it. Here the Tibetans could learn a thing or two from the Chinese and Japanese masters. But this likely won't happen any time soon, since virtually every Tibetan Buddhist has already been thoroughly convinced by their tradition's self-proclaimed superiority.
BTW, no one said it was impossible to become a buddha in this life. The notion is "is buddhahood in a single lifetime" a possibility? In India Mahāyāna, the answere is no-- Indian Mahayāna of whatever stripe requires three incalculable eons at minimumfor full awakening. In Vajrayāna, the answer is yes, since through special methods it is made possible.
It's a flimsy argument. There is no reason to accept that the vajrayāna rhetoric of progressing from a common person to complete buddhahood in one lifetime is anything more than a provisional teaching. Firstly, the numerical disproportion makes any literal, direct equivalency absurd. We're to believe that the path of method is so extraordinarily efficacious that meritorious activities and supramundane knowledge which would normally take a minimum of three incalculable eons to accumulate can magically be achieved in the span of one human lifetime. And second, the description of the abilities attained upon entering just the first bhūmi are so cosmically mind-blowing that to interpret this bhūmi model literally, one will have already progressed from an average Joe to a cosmic superhero merely upon entering the first bodhisattva stage.
Much to their credit, the Chinese and Japanese Chan/Zen masters were pragmatic and sophisticated enough to not take these Indic archetypes too literally. For example, the Zazenron states:
Q: Do Bodhisattvas of the Mahayana also have this way of no-mind?
A: Bodhisattvas have many defiling and obscuring obstacles in their consciousness and do not yet accord with no-mind until they reach the tenth Bhumi. These “defiling obstacles” mean that, until the tenth bhumi, they still desire to seek the Dharma, and they do not accord with their original way. It’s only when they arrive at the tenth Bhumi and the virtual enlightenment that they arrive at the way of no-mind.
Q: If it is so difficult for even a Bodhisattva to accord with, how then could beginners so easily accord with this way?
A: True Dharma is inconceivable. The setting up of the Bodhisattva path is for those of dim spiritual vision. Those who are clear-sighted realize the true enlightenment of no-mind when they first give rise to the aspiration.
Q: One who see’s one’s true nature and awakens to the Buddha way is called a Buddha. Why then do they no also have psychic powers, show radiant lights, or perform the mystic feats of the Buddha which would distinguish him from a regular person?
A: Since this body has been built from ignorant thoughts from the past, even though we see our nature it does not show off the psychic power and radiance. Yet, is it not psychic power to be master over the six dusts of the senses and deluded thoughts? Without resorting to hard & painful practice, without passing through the 3 great incalculable eons, to cut off birth and death, see straight into one’s nature and become a Buddha - this are the mystic feats [of a Buddha]. And to employ the light of prajna that is the pure Dharmakaya to save all beings from the darkness of delusion - what other kind of radiant light do we need? To desire psychic powers other than great wisdom and understanding is the way of Mara and the infidels. Even foxes have these psychic powers and ability to transform themselves - but should we pay homage to them? Just cultivating no-mind, we can extinguish at once the three incalculable eons and suddenly see our nature, becoming Buddhas.
And on the notion of accumulating merit over three incalculable eons, the same texts says:
Q: But if we don’t gain merits and plant good spiritual roots, how can we arrive at Buddhahood which is endowed perfectly with the various virtues?
A: He who seeks Buddhahood by gaining merit and planting good spiritual roots might become a Buddha after 3 kalpas, but one who cultivates the direct pointing at one’s own mind, seeing into one’s nature and becoming a Buddha, knows that one is a Buddha from the very beginning. It is not that he attains the fruit of Buddhahood.
Here we have the same argument of the qualities being present in the basis.