Which units would you replace in the Dark Elf faction?

I think everything should stay as it is, but it would be good to remove the FAT Hydra. In my opinion, it is practically useless. It moves rarely. The turn time is very long. The only good thing is the attack with all heads.

The Hydra should definitely not be removed; it is a classic and very much needed. Without it, the Dungeon faction is nothing. It's cheap cannon fodder.
In general, I would probably remove the Riders. Instead of them, I would add poisonous creatures that shoot with poison (like in the fourth Heroes game). I think it would turn out well.