For the most part we aren’t always aware of who is an enemy of God, and if in doubt we should love them. However on occasions we find believers, like David, who wrote a number of imprecatory psalms calling for judgment upon his enemies, not because he disregarded the command to love our neighbour, but because he was conscious that these were not just his enemies, they were God’s enemies.
While normally we need to exercise caution in pronouncing someone the enemy of God, when it comes to Satan, I don’t think we are in any doubt. We are called to “Abhor that which is evil” (Rom 12:9), and I’d suggest that includes the evil one.
After all, we know there is no repentance for him, because hell itself was prepared for the devil and his angels and Rev 20 tells us he will be cast into the lake of fire.