The devil is known as the "god of this world." He was your spiritual father before you joined the family of God (John 8:44, Ephesians 2:2). Yeshua called the devil a thief, who came to kill, steal and destroy (John 10:10).

The way to overcome him and his demons is to make sure you are outfitted with the spiritual armor of God listed in Ephesians 6:10-20.  Become intimately familiar with it. Sleep in it. Never take it off. Bind the two-edged sword to your hand so that you never lose its grip.

Another enemy that we often need to overcome is ourselves.  These fleshly bodies that house our spirits often become accustomed to indulgences that don't always reflect the righteousness that Yahweh would have us express.

The Bible is very clear regarding how to keep ourselves spiritually clean the way He wants us to be.  It says;

Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.
Phil 4:8