when she asks for his advice, his opening line is:

"i never thought that dragons would exist again...no one did"

then he says "the people who follow you know you made something impossible happen. maybe that helps them believe that you can make other impossible things happen"

then he says "but if you use them (the dragons) to burn castles and melt cities, you're not different, you're just more of the same."

so, he's saying that she can't go just melting King's Landing because the people won't support her, but she can use what she's done (birthing and using dragons) to inspire and get her people to be strong behind her.

and really, isn't defeating the entire Lannister army in about 2 minutes something most people would think...impossible?