Credits to Youtuber titus huang for his solution, I would not be able to complete this LHB without his guide:
Unfortunately my Brave Lyn is -Atk, so this is a slight variation of the solution he used (utilizing Spur buffs to make sure Lyn gets the kill).
Lyn also needs Desperation 2 (not 3), because she needs the calvary healer to bring her HP down to less than 50% for Azura to be able to use Wings of Mercy (this does not happen if she has Desperation 3).