Infantry is every chaff, including zombies, norscans and clanrats.
Armored are chaos warriors, chaos marauders and stormvermin.
Berserkers are berserkers(yeah i know) and plague rats.
Monsters are bosses.
skaven and chaos applies to every mob from said faction including specials and bosses.
I havent found anything that increased dmg on specials besides skaven and chaos
As for marauder yeah i mean big naked guys with silly black helmets and 2 handed axes. I seriously cant be bothered to remember them since they arent official units.