'Well again the romans are an example of extreme military superiority - a set of legions and auxilia which in total numbered 500,000 men with proper difficult to pierce armor against poor weapons from smaller, separated tribes conquered one at a time.'

Which time period are you looking at? You'll find that their enemies normally had decent weapons and armour too, and when they didn't, it was normally due to Roman oppression or aggression earlier.

'And I oppose to your view on the roman empire case being an exception. Surely it fell after centuries, but over that period many more romans than non-romans got slaughtered per population size.'

This seems unlikely. You also neglect that those killed of the populations of those other people are too dead to breed, and many of the survivors no longer have property, a high standard of life, or often liberty. Numbers of deaths over the entire history of the empire/republic may well be higher than for their enemies, but that was generally because their enemies ceased to exist as independent peoples in the long term.

'The romans elected the strongest of their population to man the legions, these got killed en masse. Other people did the same, but while suffering more casualties in total, suffered fewer per population size. The roman empire also caused a dramatic change in world politics outside the empire - smaller tribes which had previously had no reason to ally did so. Out of smaller Germanic tribes, the Franks and Allemanni were formed, for instance. No doubt the romans suffered more in the end.'

No. There were plenty of nations, not 'smaller tribes' at the time they started. The Celts also selected their best men. Thanks to the attrition of the Roman war machine and their own civil war, they almost all died.