This command does not accept the attribute:

/summon Guardian ~ ~1 ~ {Invulnerable:1b,Attributes:[{Name:"generic.attackDamage",Base:100.0f}],Elder:1b}

This command does

/summon Guardian ~ ~1 ~ {Invulnerable:1b,Attributes:[{Name:"generic.attackDamage",Base:100.0f}],Elder:0b}