Michael the archangel and his angels battle Satan and his demons. They overpower him and drive him away (Rv 12:7-8). Michael stands as a bulwark against Satan, a reinforced wall to block his plans (Dn 10:21—11:1). Raphael also defeats demons, overpowering and binding them (Tb 8:3).
Jesus, Whose outpoured blood defeats Satan (Rv 12:11), has given the Church power to "overcome all demons" (Lk 9:1). We members of the Church do our part by our testimony to Jesus (Rv 12:11), prayer, fasting (Mt 17:21, NAB), our steadfast refusal to yield to Satan (Jas 4:7), obedience (Rm 16:20), and our praises, which Satan and his demons cannot stomach.
If Satan has been defeated, why must we still fight him? He has lost the war, but on his way to hell, is trying to exact more casualties from the kingdom of God. He is a sore loser, and "his fury knows no limits, for he knows his time is short" (Rv 12:12). Therefore, if we "relax our efforts" in finishing the fight (cf Gal 6:9), we might suffer some casualties ourselves. Because God has used angels to successfully fight Satan, and because the Lord hasn't changed His battle plan (see Heb 13:8), we need the angels (cf 1 Cor 12:18, 21). We must regularly involve the angels, especially the archangels, in spiritual warfare. St. Michael the Archangel, pray for us.