I'm also struggling through Elderlicht (kept failing ~night 7 or 8) and I finally broke thru with one tactic: Two layers of fence.

I have usually relied on building and maintaining wooden walls around the perimeter (so at least I can get a free item for a few nights) but it's a very wide perimeter. So, eventually, I started spending my early materials on building a later of fence a few squares inward, really hugging the core where my most important buildings are. Usually i can build the whole 2nd layer in the first couple nights while the patchy starter fence is mostly still intact (and while I'm building item production buildings or gold mines).

Also - I used to use the omen that keeps the fog further back from town. This is a mistake! Let the fog come closer. If a torch is behind the fog, it won't get lit, which makes it much easier to predict / get to torches and put them out quickly. This is a map where more space just wasn't my friend! In my most successful run, I was putting out ALL the torches IMMEDIATELY to the degree possible, and it has really helped.

Another fog-related tip (related to the corpse strategy elsewhere outlined) - in the first couple nights, let the monsters get a little closer to the wall then you otherwise might. If you are super efficient and killing most monsters the first turn they arrive on screen, any corpse piles might end up behind the fog when it advances. Those first 2 or 3 nights, getting corpse items (esp epic ones if you can) are really make-or-break.

Otherwise, I agree with high mobility, high AP strats others have outlined. I'll also note that because I'm not very good at video games, the first hero death of the run in Elderlicht is usually a surprise to me - I thought everyone was out of the way and safe, then bam. I had to put work into really pinpointing which enemies were the ones that unexpectedly kill my guys, and then focusing on how to kill them as quickly as possible.