Destroy the tombstone as soon as the mission starts, and you'll learn the retreat song. It goes as Pon Pata Pon Pata (Circle, Square, Circle, Square), and quickly makes all units fall back to avoid damage. This boss is Dokaknel, upgraded version of Zaknel. It feels like Dokaknel is about as strong as Zaknel level 3, but with the help of the newly aquired song, you should be able to kill the beast if you can already kill Zaknel at level 2.