Hmm... Do weapon/exp upgrades increase lethality? They have 0.04 nominally (same as any untrained archer), so they shouldn't be able to tear through anything really, only 1 out of 25 hits should kill. I took a single unit of medium infantry against five of them (without any upgrades though), and killed about 65% of them before my few remaining guys panicked in the mass of people and were overrun.