This is SPOILER, do not read this if you no nothing of the Dragonborn DIC plot.
Don't fight Miraak head on, put on Dragon Acpect, use Bend Will to mount one of the dragons (I recommend the black one since it is already your ally) don't throw spells at Miraak, make the dragon attack him. Wait for Miraak to kill the dragon, after the dragon is dead, repeat this on the last two dragons.
Once all dragons have died by Miraak's hands, give him a beating with your fists or the weakest weapon you have.
If that doesn't work, then make your dragon attack one of the other dragons until the other dragons are dead. If your dragon died before you had a chance to kill the others, mount a new dragon and kill the dragon you've been attacking or vise versa.
Once two of the dragons are dead, make yours attack Miraak, after Miraak slays it, FINISH HIM!