So this is first boss, Gurag The Destroyer.

Attributes
-1000 health
-8 melee damage
-Normal blaze attack
-20 block follow range

Command: /summon Blaze ~ ~1 ~ {CustomName:"Gurag The Destroyer",CustomNameVisible:1,Equipment:[{id:stone,Damage:6,Count:1,tag:{display:{Name:Gurag`s Heart,Lore:[You can maybe uy something with it]}}},{},{},{id:stone,Damage:6,Count:1},{}],DropChances:[1.0F,0.085F,0.085F,0.0F,0.085F],Attributes:[{Name:generic-maxHealth,Base:1000},{Name:generic.attackDamage,Base:8},{Name:generic.followRange,Base:20},{Name:generic.knockbackResistance,Base:100}],HealF:1000,PersistenceRequired:1}