The biggest thing is avoiding charging up the Dark Lord and his special firing. So it’s mostly about killing the two side enemies, and then meticulously ghosting tiles on the sides without hitting him much or ever, and using your special skills to kill him.
It’ll be a slow battle, but that’s the easiest way to do it.
Mana management helps a lot too on this front, really regardless of color. But you happen to have Chao, Li Xiu and Onatel as potential options (if leveled) who are also strong color.
Like @littleKAF and @PeachyKeen both said, further leveling will help — though it is certainly doable with some care and luck, like @PeachyKeen said.