Though this is mostly specific to Mannpower, it's possible to survive the supposed one-shot of the Half-Zatoichi if you're also wielding the Half-Zatoichi. A simple fix is in tf_weapon_sword.cpp at line 562 to increase fDamage from pTFPlayerTarget->GetHealth() * 3 to a larger value.
Though this is mostly specific to Mannpower, it's possible to survive the supposed one-shot of the Half-Zatoichi if you're also wielding the Half-Zatoichi. A simple fix is in
tf_weapon_sword.cppat line 562 to increasefDamagefrompTFPlayerTarget->GetHealth() * 3to a larger value.