As 8BitCrab says, there are some json files that you can change to make this happen.
If you haven’t already, you should, first of all, make sure that you can look inside the stonehearth main mod. You can do this by copying the stonehearth.smod file to some other place, rename it to stonehearth.zip, and extract the files in there.
Now, to change the attributes of the giant zombie: it is created using the original zombie definition file as a base (found in entities/monsters/undead/zombie/zombie.json), but it’s modified when spawned through the ambient threats campaign to make it bigger and such. The file in question is found in data/gm/campaigns/ambient_threats/arcs/trigger/ambient_threats/encounters/create_zombie_chest_camp.json.
Once there you can find the values for the giant zombie within "boss". There are a few values that you can change in order to make it harder: give it a new weapon, so it can spawn with something more deadly; increase its muscle attribute, which increases the melee damage caused by the mob; increase the max_health attribute, to make it more durable.
If you’re wondering at how to make a mod to begin with: take a look over in this thread to help you get started. You also should know how overrides and mixintos work.
If you’ve got some more questions, you’re always welcome to ask them. Good luck!