How do demons win in fighting against angels and how do angels win?
I would like to give you a Catholic perspective. The Catholic Church teaches about what is called the Beatific Vision – seeing God in all his essence. This is taught also in 1 John 3: 1 – 3. When we see Him we shall be like Him. Anyone who sees God in His essence would never ever sin. We shall be like Him. That is why there is no fear that we would ever fall way from Him after He returns. We shall see Him as He is and no one who sees Him in all His goodness would ever desire to sin against Him. He would have fallen in love with God. He will see God as the fulfillment of all his longing.
So in the beginning, the angels went through a time of probation. They saw God only in a limited way. This was a test to see if they would follow God. Lucifer, because he did not yet have the Beatific Vision of God, thought that he could with other angels rebelling against God and win. He now realizes that this could have never happened. Satan and the demons have lost the war.
The only thing they can now fight over is souls. They can win a battle if they can tempt a soul to disobey God. That is the only way they can hurt God. His love for us cannot be overestimated. Satan is seeking to hurt God as much as he can, and that is by bringing souls to his side. God is just. He cannot allow us to go to heaven, no matter how much He loves us, if we choose to side with Satan. He would have to send us to hell to be with Satan. It is not something outside of Himself that is forcing Him to this. It is within His holy nature.
God could have told all the angels on His side to just sit back and He will fight for us. And God sometimes does this. But He is pleased to work through the angels. They will fight for us but not with conventional weapons. We can help them help us – through our prayers. The more we pray, the more the angels are give the power to overcome the demons in our lives.
What happens when one side wins?
God’s side has already won. It was never really a war to begin with. All the demons are nothing more than ants in the palm of His hand. The demons can only hope to hurt God by causing more souls to follow them in hell.
Do the other die and perish? If they don't, as it is traditionally believed that spirit being never "perish" or disappear, how then can there actually be any won fights? For since both side has immortality, they can fight forever?
No one dies. There can be battles won over individual souls. But Satan will eventually be cast into the lake of fire, along with his demons, and any souls that chose him over God.
But God is greater than all, and even if all the forces of the universe come against Him "it is like a drop in a bucket" for Him to deal with. Blessed be our God and Savior, who gives us the victory through Jesus Christ. Even if the world stands against us, let us, by His grace, stand unwavering in Christ, until He returns or calls us home. Amen! God's love be with you.
Amen! And may God’s love be with you