has entered now phase 2 in which Plague becomes more "dragon" like with an emphasis on fire attacks.
This illustrates well how it would have gone down in the comic as well. Twilight and her friends would fight Plague at first, get them selfs to the brink of defeat, Celestia and Luna would intervene in the last second and then move the battle to the moon in order to pay time for the mane six to talk things out and reactivate the elements of harmony. However…
If this was an actual game, here is the guide for this phase:
In contrast to phase 1 this form is immune to freezing spells. Additionally Plague will spawn these minions which will randomly attack you.
Don't waste any time on the minions. Plague will only respawn them if they go down. Ignore them, focus all your efforts on Plague's main body and keep an eye on your healthbars. This strategy will help you through while you are still fighting with Twilight and her friends.
As soon as the battle shifts to Celestia and Luna, it will become slightly easier, as this part is more of an "interactive story element" than an actual fight. That however doesn't mean you should get sloppy because even Celestia and Luna can get a GAME OVER.
What you have to do is keeping them alive until their limit bars are full. Then you have to choose their limit breaks as quickly as possible to unleash them on Plague right behind each other, with Plague being unable to interrupt. If you can manage to do that it will end this phase and you will finally get to phase 3.