my personal styles

got 2 healer, or at least mobs that can disturb HP healing like fire harpu, fire grim reaper, fire succubus or wind mystic witch

just play tough, it take times but slowly kill the crystall 1st.

while u take times to kill the crystall, dont forget to heal ur team HP.

without the right side crystal, the boss cannot healing 
my hunble not so famous team
wind sylphid (leader) + wind yeti (healer) + fire harpu (healer no2 and also buffer) + fire grim reaper (attacker) + wind mystic witch (attacker)
my team got 3 mobs that can disturbing boss healing. can take turn to kacau his healing 

my team may not 100% success rate (due to 4* and rune)
but i think around 5 round, i will fail 1 round.