Walkthroughs for custom maps.

"The Devil is in the Details" - is this the largest map ever created for Heroes, and has anyone actually completed it? I just need to know if it's even possible to beat it. I'm getting really frustrated, and I've been trying for two weeks now.

Hmm. The map is available and being discussed here. Why not post your questions there? The author or other players will answer you. There are already 98 comments in the discussion; have you tried reading them? Not knowing the language is not a problem: a translator is built into Chrome, and there is also a separate one here (you can translate text from the clipboard, a page at a given address, or upload a document).
Personally, I haven't installed HotA yet, and I haven't played any of its maps, even the ones that look very appealing.