2K HP MONSTERS ARE SUMMONED.

where can I actually find the save file?

The Games folder, which is located in the directory where you installed "Heroes." If you haven't saved the game, there will be only one file there: AUTOSAVE.GM1.

I'm just a newbie.

In that case, it's best to start with the original version (SoD/Complete Collection) - its executable file is in the same folder, and the file name is Heroes3.exe. I know from experience: I started with WoG, and it spoiled me, but then I switched to SoD, and now everything is much better.
Please attach the save file to your message (paperclip in the advanced editing mode), and we'll take a look. First, you need to check and probably change the WoG Options: there are many parameters there, some of which affect the growth of AI enemy armies and the experience gained by their units.
And also: what map are you playing? Judging by the screenshot, where part of the mini-map is visible on the right, it's one of the standard maps. In that case, it's highly undesirable to play it in WoG - you will hardly be able to win, let alone learn how to play. You should switch to WoG only after you have mastered SoD.