My own understanding of that text is that what is present on earth, a mysterious presence called 'Wicked', shall be consumed ('annihilated' is quite a good translation of αναλισκο, *analisko* [Strong 355][1]) during time, by 'the spirit of his mouth'.

Then, at the end of time, that same Entity will be fully destroyed by the brightness of the glorious return of Jesus Christ to earth, that is to say a new earth. But now we see, not yet, all things put under him, Hebrews 2:8.

At present, Christ rules 'in the midst of his enemies', Acts 2:32-35, his enemies being yet suffered a place. But at the end of time, it shall be given to Jesus Christ to dash his enemies to pieces as an iron rod does to a potter's vessel, Psalm 2:9 and Revelation 2:27.

The 'spirit of his mouth' denotes, to me, what can be referred to as 'his mouthpiece'. It is as though Christ is present on earth, bodily, speaking certain words. 

But he is not, yet, present. There is that which represents him. That is to say, the Christian ministry, those such as Timothy or Titus who are called with an especial vocation to preach and to teach. In the book of the Revelation such are referred to as 'the two witnesses', Revelation 11:3, and their activity, in this same context, a prophetic witness against earthly evil, may be seen there.

Thus the spirit which goes forth from the mouth of Jesus Christ in heaven, also goes forth on earth by his mouthpieces, to the temporal annihilation of the adversarial Wicked in front of the audience of those who dwell on earth.

This will happen repeatedly, generation by generation, as we see the two witnesses repeatedly killed, their murderers exchanging presents in triumph, only for the two witnesses to rise again. That is to say, another generation of witnesses rises to testify, acting as the mouthpiece of the risen Christ.

The prophets may be seen as performing a similar role. One thinks of Isaiah in particular who, by his words, effectively 'demolished' the adversaries of the Lord, shaming their opposition and their reputation.

At the end of time, by the brightness of his coming, the Lord shall eternally destroy the opposition of the Wicked One.

What exactly that 'Wicked' actually is, belongs in another question.






[1]:https://biblehub.com/greek/355.htm