Don't bring a knife to a gunfight
"Don't get caught with your pants down."
(Assuming lack of sufficient preparation is the key aspect desired, this is about the softest analogy I thought of.)
Like …bringing pennies {or pocket change} to a high-stakes poker game
would be a less violent, perhaps “softer” way to express the idea.
(from ‘The Big Picture: Filmmaking Lessons from a Life on the Set’ By Tom Reilly, via Google Books)
short and direct: you can't put out a fire by peeing on it (answer inspired by the writings of Jonathan Swift)