Q: Krishna was not a Sannyasi or a brahmachari, then how is he qualified to give spiritual advice?
It is interesting that the householder Krishna advises a householder Arjuna on how to lead a life towards real Sannyasa. Sannyas is not running away from life, but living your life fully dedicated to the society in a spirit of detachment. Live with complete happiness in spite of all obstacles you may have to face in life is real Sannyas. Krishna lived a life that was an embodiment of what he preached.
When we take the examples from Sri Krishna’s life, we can see that he had to face a multitude of hurdles even in his childhood. Six of his siblings were thrashed to death by his own uncle. Sri Krishna was born in a jail and was attacked by demons many times. But even during all these, he lived fully established in happiness. Not allowing any of these external circumstances and hurdles to affect his equipoise. Finally, towards the end of the Mahabharata, his whole family fought and killed each other. Even while leaving his mortal frame, Sri Krishna had a smile and was completely happy. Therefore, his life is a perfect role-play of the greatest ideals that he had preached.