Sunday punch 
noun informal, chiefly US
A powerful or devastating punch or other attacking action: the Belgian unleashed his Sunday punch to knock out the champion...
- All were welded into one compact, devastating fist, set to deliver the Sunday punch.
- All over the U.S., all last week, the Sunday punches were thrown in the last round of the 1954 political campaign.
- Where I come from, you save your Sunday punches for the general campaign and I just don't think Rudy will withstand the heat in the general election.