* NPC hurt you with:
Angel Cartel - Kinetic / Explosive / Thermal
Blood Raider - Thermal / EM
Guristas - Kinetic / Thermal
Sansha's Nation - EM / Thermal
Serpentis/Sevens - Kin/Therm
Mercenaries - all 4
Drones - all 4 - kin/therm
Amarr Navy - EM / Thermal
Caldari Navy - Kinetic / Thermal
Gallente Navy - Thermal / Kinetic
Minmatar Fleet - Thermal / Explosive
Ammatar Mandate - Thermal
Thukker Tribe - Thermal / Explosive - Explosive Missles

* NPC are weak to these damage types:
Angel Cartel - Explosive
Blood Raider - EM
Guristas - Kinetic
Sansha's Nation - EM
Serpentis/Sevens - Kinetic
Mercenaries - Thermal
Drones - EM / kinetic
Amarr Navy - EM / Thermal
Caldari Navy - Kinetic
Gallente Navy - Thermal
Minmatar Fleet - Thermal
Ammatar Mandate - Thermal
Thukker Tribe - Thermal

Stacking penalties
f(1) = 1
f(2) = 0.8708860
f(3) = 0.5705831
f(4) = 0.2829

You're welcome.