a big gun
1. The biggest or most powerful tool or asset for a given task. Usually plural. This axe is getting nowhere on this stump—it's time to bring out the big guns and use the dynamite. My phone calls have not been answered, so it's time to bring out the big guns and send them a subpoena.
2. An important, successful, or influential person. He's a big gun at the law firm; he wins every court case he gets. After failing to convince the IT department that implementing new network security controls would be in everyone's best interest, Mike felt it was time to bring in the big guns, so he called a company meeting with the executive board.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a big gun
COMMON If you call someone a big gun, you mean they are a very important and powerful person in an organization or area of activity. He's a pretty big gun in Maine politics. Note: Often, people use the plural the big guns, meaning a group of important and powerful people. She has been much sought after by the film industry's big guns. The Premiership big guns are all keen to get their hands on this young player. Note: Cannons used to be referred to as `big guns' or `great guns', while rifles or muskets were called `small guns'.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012