The Hebrew word שָׂטָן, transliterated as "Satan," means an enemy or one who opposes and resists us. The Greek word διάβολος (diabolos) usually translated as "the devil," means a slanderer or one who accuses us. And so it's not surprising that this passage in Revelation describes our enemy as "the accuser" of the believers, and says that he is busy accusing us before God, both day and night. Sometimes he accuses us through other people, sadly even sometimes through our Christian brothers and sisters. But his most usual way of accusing us is through our own thoughts: memories of failure, feelings of inadequacy, awareness of sin will all be exploited by the enemy to accuse and condemn us. That's why a key strategy for us is the one described in 2 Corinthians 10: 3 - 5. We need to learn how to "take our own thoughts captive," so that the enemy can't use them to bring us down.
This Revelation passage mentions two more weapons in our armoury: the blood of the Lamb (our authority to proclaim the death and resurrection of Jesus) and the word of our testimony (remembering and speaking out the ways that God has been there for us and has triumphed in the past.) It's probably also key in this passage, that the believers "didn't love their lives" and were willing to die for their faith. But perhaps it's easier to die a heroic death than to live with a hurtful accusation or slur on your character/reputation, and this may be why accusation is still the enemy's most powerful weapon against us.
Reading this passage a couple of years ago, I got to thinking and meditating on why two such different weapons are mentioned… and my theory is that it's because there are also two different kinds of accusation from the enemy. Very often the enemy's accusations against us are blatant deception, not even based in truth. Jesus Himself called the devil the "father of lies" (John 8:44) and said that there is no truth in him. The enemy will lie to us, telling us that we're not good enough, that God doesn't love us, or that our prayers won't be answered. It's at such times that we need to call forth the word of our testimony and proclaim the truth of how God has been faithful and has answered our prayers and used us to win victories in the past. The Word of God (the Bible) and the word of our own experience of God are powerful weapons against the enemy's lies.
Just occasionally, though, the enemy's accusations are true: he'll tell us that we've sinned, that we're weak, or that we're lacking in faith. These things are probably true, and so that's when we resort to our other weapon, the blood of the Lamb. We can remind the enemy that, even though we are sinners, we have been cleansed and forgiven because of Jesus' death. We can proclaim that even though we're weak, we can do all things through Christ who strengthens us. And we can rest in the fact that winning the war does not depend on how strong or weak our faith is, but rather on how strong and powerful God is.
These are the things, together with our praise and worship of the living God (even the praise and worship of little children; see Psalm 8 vs 2) that will silence the enemy's accusations and cause him to flee from us. Let's remember that much of the battle takes place in our mind and thoughts. If we take our thoughts captive, draw near to God and resist the enemy (James 4: 7) our accuser will need to flee from us.