Had the same problem in my recent Rome campaign and found the issue to be AI's urge to always attack your strongest/biggest unit if possible. They always concentrate all missile fire in range on those poor legionaries or send more infantry their way. That being said, I feel in a 1vs1 they fare just as good as their comrades.

This is due to HP system that TW games are now using.

Basically, there is total unit HP pool which is amount of soldiers x HP entry.

While you cant see it in Rome 2, you should know how it works if you played Warhammer or later games.

When unit is attacked especially by ranged, it does not keep perfect track of how much HP each solder has left, instead it uses total HP and then randomly applies deaths.

The smaller unit size is, the more time it will take for soldiers to start dropping dead. This is why you often can see elephant take no casualties and then in few seconds most of them are dead.

I had a great example in Warhammer, in most extreme scenario, I took 60% of total HP of Greatswords before first one got killed and then next volley dropped like 25 guys.

In terms of double stregth units like First Cohort, they have exactly twice as much total HP as regular versions of them but due to that game has more leasure to distribute deaths.

Thanks for the clarification kam2150! That actually really helps understand whats going on with them and other special/high HP unit types.

So does that mean that the HP pool has to deplete before the unit starts taking casualties or are casualties assigned as the pool is depleted?

It is basically one, big RNG. Just total % of lost HP increases chance for individual models being picked off, the more models, the higher the chance.

I've never seen a rule to it, outside of model count. Like in Warhammer, I sometimes see units not dropping until they really lose a lot of HP and sometimes there are people getting killed much sooner.

I guess that's why so many units are at 200, it's kind of a sweet spot of having enough meat, and not too many troops as to lose them in such a fashion as you've described.