In the second encounter, the manual says that overlord can move lord farrow 1 space for each fatigue token gained in Encounter 1.
I haven't understood if this movement is made only during preparation and then lord farrick will move regularly (with his speed of 4) or not.
Anyway i think that this mission is quite impossible for heroes and now i'll try to explain the reason.
In first encounter even if the heroes are fast, they will be blocked by dragons or other big monsters in the first room, and so they need normally at least 4-5 turn to reach lord farrow. This means that, with all the monsters near lord farrick, OL need to roll 2+4 or less (we're playing with 4 heroes, so there are 4 Flesh Moulders) to gain fatigue, which it's pretty easy with black and grey dices (he can fail only with 7 or 8). So let's say that after the first encounter, lord farrick will have 4,5 or 6 fatigues in most cases.
When second ecounter starts, if i understood well the rules, farrick will move 4,5 or 6 spaces so. Easily he will reach the first open door. Then the OL will place all the monsters. The scenario also says that , no matter what, the heroes will always skip their first turn. So it's pretty clear that after the first OL turn, the heroes will have to fight against all the flesh moulders and the first group of monster (in our case, dragons…) and it's impossible for them to move across them, so they'll have to fight at least for 2-3 turns (even more if there are dragons because of the "shadow" rule to hit them).In the meanwhile, just after the first turn, Farrow will be near the second door (cause he can move trough monsters of course). So even if Merrick fails to open the second door, he'll have at least other 2-3 turns to succeed (and it's not so hard to open the doors with his stats). Anyway he can also use double movement and OL has also the card to move him even more (i don't remember if it is called "dash" or something similar).
With all these conditions, i think that only in he case of very bad luck for OL with die, the heroes can have a chance to reach lord farrow and maybe win, otherwise it seems impossible.
What can you say about it? Have you tried this mission?