Experience shows that only Ghosts prove unbreakable on the first. Vampire Lords, Archmages, and Battle Trolls look tough, but they're usually fewer than 15. They can be broken through with troops from pickups. I've broken through 20 Strong — Grand Druids without problems (and not with a Might hero or without magic), that seemed to be the count; the rest of the walkers pose no issues at all. Speaking of pickups, by the way, it's often advantageous to play as Knight for Green, especially if several heroes appear in a row (if any readers are just starting to master online — look at your leisure at the devastation a hundred Peasants can inflict on neutral ranks when used wisely). And for Yellow too, given the motley nature of the company, you need to grab the lead as quickly as possible if a Barbarian is chosen. Since the key stack is usually 1 (Hobbits for Green, Archers for Yellow), even minimal negative morale can put you in a deep hole; I fully agree with Captain. When fighting low-level units against neutrals of level 4-5, which are usually at the passages, morale -1 will cause trouble every other time, or even more often, so when forming a mixed stack, I carefully watch which way the bird's wings tilt.
Teleport guards are perhaps the most problematic aspect of this map. Black Dragons, Titans, Phoenixes are certainly unpleasant, but I think none of them compare to Genies.