I got to "Marshal" on "Hero" difficulty and got completely stuck. I guess my Railag isn't leveled up enough, but I can barely get through the first ghosts, and then G. is there and kills me immediately. Do I really have to replay the entire campaign?

Chaos Magic is the most important weapon for a Warlock. The developers designed this mission around Chaos Magic (Summoning). Good spellcasting is ensured by the "Education" skill, and a good amount of mana is provided by the "Magic Attraction" skill in "Education." If you don't have these skills, you probably won't be able to complete this mission, or the next one :(.

As for "G.," I don't understand. He doesn't seem to attack at first, but runs away. One option is to gather wax in the dwellings, of which there are plenty on the map; then you can try to overwhelm the "dog handler" with a living mass :). But only on "Recruit" difficulty :).