shoot someone down in flames = shoot something down in flames
forcefully destroy an argument or proposal.
1999 - BBC Top Gear Magazine - I’ve been shot down in flames by the boys in the Top Gear office for saying this, but I reckon the Porsche Boxster has to be one of the most beautifully sculpted bits of artwork going.
RELATED IDIOMS :
run something up the flagpole
test the popularity of a new idea or proposal
The idea behind this expression is of hoisting a particular flag to see who salutes.