Here is the context:
4 For certain individuals whose condemnation was written about long ago have secretly slipped in among you. They are ungodly people, who pervert the grace of our God into a license for immorality and deny Jesus Christ our only Sovereign and Lord.
Jude complained against these impious men who deny Christ.
8 In the very same way, on the strength of their dreams these ungodly people pollute their own bodies, reject authority and heap abuse on celestial beings. 9But even the archangel Michael, when he was disputing with the devil about the body of Moses, did not himself dare to condemn him for slander but said, “The Lord rebuke you!” 10 Yet these people slander whatever they do not understand
These were Impious and ignorant people. They did what they did and said what they said due to their impiety and ignorance. We are not to imitate them.
Does this mean the Devil is an "angelic majesty"?
Not necessarily because these were words of the ignorant.
Somehow we might pronounce a "railing judgment" against him?
No. We are not to imitate these ungodly and ignorant people. We are to imitate the practice of Michael the archangel.