Tier 9 - Minos
Tier 11 - Poseidon
Flagos only method of dying is Flagos to Flagos combat by way of destroying their core
@cfatalis: Retrias is likely Tier 6. His control over metal is hax an the possiility of wiping all life is present but that's just as it is. He's relatively slow and I wonder whether he could normally survive a nuke without considerable preparations.