*** NPC damage types ***
Guristas - Kinetic Thermal
Serpentis - Kinetic Thermal
EoM - Kinetic Thermal
Blood Raiders - EM Thermal
Sansha's Nation - EM Thermal
Angel Cartel - Kinetic Explosive
Minmatar Republic - Kinetic, Explosive Thermal
Mercenary - EM, Kinetic, Explosive Thermal
Rogue Drones - EM, Kinetic, Explosive Thermal
Mordus - Kinetic, Thermal

*** Best damage types to NPC ***
Guristas - Kinetic
Serpentis - Kinetic
EoM - Kinetic
Blood Raiders - EM Thermal
Sansha's Nation - EM Thermal
Angel Cartel - Explosive
Minmatar Republic - Exp
Amarr - EM / Thermal
Gallente - Thermal
Caldari - Kinetic / Thermal

Stacking penalty mod
1st: 100%
2nd: 86.9%
3rd: 57.1%
4th: 28.3%
5th: 10.6%
http://eveinfo.com/missions/4/