The explaining for the mechanism is in
https://wowlazymacros.com/forums/topic/the-true-lazy-macros/page/2/#post-3852
Less than 3 target tank
/targetenemy [noharm][dead]
/castsequence [nocombat] reset=0 0,ice touch
/cast [nocombat] outbreak
/castsequence reset=3 death strike,heart strike,rune strike,death strike,blood boil,rune strike
/cast bone shield
/startattack
More than 3 target tank
/targetenemy [noharm][dead]
/castsequence [nocombat] reset=0 0,ice touch
/cast [nocombat] outbreak
/cast [mod] blood boil
/castsequence reset=0.3 1,Horn of Winter
/castsequence reset=3 death strike,blood boil,rune strike
/cast bone shield
/startattack
Blood shield should never drop! (unless the boss smashed your face)
Have fun