Set the weapon damage as kill_pilot or something. Look at Jarmen Kell weapon damage type. It will state it there. Than add that damage type to your weapon or secondary weapon.
1. create a PilotSniper weapon like JarmenKell's with attack range smaller than poison field's.
2. give the poison field object a FireWeaponUpdate to use the weapon created above.
Wouldn't that make the poison stop killing infantry though? In armor.ini it has everything but vehicles immune to poison.
I suppose you could change the human armor to also take damage from kill_pilot, but I wonder how well that would work, and how fast damage would be dealth.
Unless there's a way to give something two damage types?