I took out the demon using 2 teams of archer/witch - Nanaly and Belinda top right corner with a healer (Iris) and one lower left (Spica/Adele). Let him run the gauntlet and he dies easy.

Once he is past the top right team, drop out a heavy duelist (Imelia) and she cleans house and lightning rods as the flyers distract the Archer/Witch. Prince right bottom corner out of harms way to keep everyone hitting hard. Drop healers/other archers/Elizabeth and withdraw units as necessary. I tried sticking Flamel behind Imelia and that worked well too.