Any ideas for a saying that means the opposite of using a sledgehammer to crack a nut? Basically I need something that means if the problem is big, then don't underestimate it, and make sure you have the right tool
The answer is in the question : 'the right tool for the job' sometimes (as in the question) stated as 'the right tool for the right job'.
One of my grandfather’s favorite sayings was “Use the right tool for the job”—common-sense advice that applies to a wide range of situations.
Relevant to the context of the question is :
Communication : Choosing the right tool for the job
Go in loaded for bear
loaded for bear phrase [North American] [informal]
Fully prepared for any eventuality, especially a confrontation or challenge.
‘as my ancestors were wont to say, watch out, John Bull, I'm loaded for bear!’