The adjective “great” refers to how frustrated the devil is and so how intimidating and overwhelming and dangerous his anger is. The shape of this anger receives further description in the following verses/chapters. For now it is enough for earth and sea to know what is so woeful about the seventh trumpet (in escalation of the woes of the fifth and sixth trumpets): now the dragon is let loose upon this planet in hellish rage. He is desperate precisely because his time is short.
12 Therefore, rejoice, O heavens and you who dwell in them! But woe to you, O earth and sea, for the devil has come down to you in great wrath, because he knows that his time is short!”