What is the ruling on guardianship and assuming it through a dream, as some people claim?

If you mean that sainthood (wilayah) is established by a dream, such as someone being told in a dream, "You are one of Allah's saints," then know that sainthood is an acquired status attained through faith and piety, as Allah Almighty said: "Unquestionably, [for] the allies of Allah there will be no fear concerning them, nor will they grieve. Those who believed and used to fear Allah." As for sainthood being established merely by a dream, no.