big gun
1. (usually used in the plural) The biggest or most powerful tool or asset for a given task. 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.