here is the command -

/summon Slime ~ ~1 ~ {CustomName:"Slime King",CustomNameVisible:1,Size:50,Equipment:[{},{id:leather_boots,Count:1,tag:{AttributeModifiers:{AttributeName:"generic.maxHealth",Name:"generic.maxHealth",Amount:25,Operation:0,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.knockbackResistance",Name:"generic.knockbackResistance",Amount:200,Operation:0,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.movementSpeed",Name:"generic.movementSpeed",Amount:.2,Operation:0,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.attackDamage",Name:"generic.attackDamage",Amount:15,Operation:0,UUIDLeast:894654,UUIDMost:2872}],Unbreakable:1,ench:[{id:0,lvl:100},{id:1,lvl:100},{id:2,lvl:100},{id:3,lvl:100},{id:4,lvl:100},{id:7,lvl:100},{id:8,lvl:100}],display:{color:5570426}}},{},{id:diamond,Count:20,tag:{AttributeModifiers:[{AttributeName:"generic.maxHealth",Name:"generic.maxHealth",Amount:5,Operation:0,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.knockbackResistance",Name:"generic.knockbackResistance",Amount:200,Operation:0,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.movementSpeed",Name:"generic.movementSpeed",Amount:.2,Operation:0,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.attackDamage",Name:"generic.attackDamage",Amount:20,Operation:0,UUIDLeast:894654,UUIDMost:2872}],display:{Name:Attribute Diamond}}},{id:diamond_helmet,Count:1,tag:{AttributeModifiers:[{AttributeName:"generic.maxHealth",Name:"generic.maxHealth",Amount:25,Operation:0,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.knockbackResistance",Name:"generic.knockbackResistance",Amount:200,Operation:0,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.movementSpeed",Name:"generic.movementSpeed",Amount:.2,Operation:0,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.attackDamage",Name:"generic.attackDamage",Amount:10,Operation:0,UUIDLeast:894654,UUIDMost:2872}],Unbreakable:1,display:{Name:Slime King's Helm},ench:[{id:0,lvl:1000},{id:1,lvl:1000},{id:3,lvl:1000},{id:4,lvl:1000}]}}],DropChances:[0.085F,2.0F,0.085F,1.0F,2.0F],Attributes:[{Name:generic.maxHealth,Base:800},{Name:generic.movementSpeed,Base:.5},{Name:generic.attackDamage,Base:20},{Name:generic.followRange,Base:100},{Name:generic.knockbackResistance,Base:1000}],HealF:800,ActiveEffects:[{Id:8,Amplifier:15,Duration:2e+47,ShowParticles:0b}]}



        Enjoy the big slime boss!!!