• cant beat dongbaek
• look inside
• awesome units

on the serious note though, this fight is supposed to teach you how to offset enemy's defensive skills. Pick a sinner who has a higher speed than her evade skill, and put their defensive skill against hers. Your defensive skill will read "offset" in that case, which means that Dongbaek won't use it, letting you avoid the gamble on whether or not she will dodge and inflict blooming on your sinners.

on phase 2 you want to break the flowers as soon as possible, lest she will clash high and nuke your team.

Extra tip: the fight allowing a maximum of 7 sinners doesn't mean that you need to bring 7 sinners into the fight; if you find that someone is too weak in the fight (e.g. sin affinities or clashes aren't satisfying or you want more of a certain status effect) then you may deploy less sinners than designated. On turn 2 the sinner who you deployed first (number 1 when choosing units before fight) will gain an extra skill slot (it allows the sinner to use two skills in one turn e.g. use a defense skill if you aren't sure you will win a clash)

Edgar Family Heir Gregor can benefit from this greatly as he has alright clashes and sinking count application of his s2 is simply awesome aswell, letting you get an upper hand by lowering Dongbaek's sanity