Satan is cast down and overcome by our testimony

"Now have come the salvation and the power and the kingdom of our God, and the authority of his Christ. For the accuser of our brothers, who accuses them before our God day and night, has been hurled down.

They overcame him by the blood of the Lamb and by the word of their testimony; they did not love their lives so much as to shrink from death."
Revelation 12:10-11

Our testimony together with Christ's blood is what will bring Satan down.