This is mentioned in some of the earlier pages I believe if you want to dig for specifics, but what I remember is that it's due to the spiders in the upper middle left near the house where you recruit Ewan that cause this. I think it's because some enemy got misplaced and so one of the spider's scripted move can't be completed and thus crashes the game.
From what I remember on how I got through this, is you can either make a beeline for the spiders using a strong flying unit if you have one that can kill them quickly enough (you could use an unlimited move/turn AR code and then turn it off once completed), or try saving at different points to where you still get the randomized units, and then use that save in a vanilla rom to complete most of the level, then use that save back in the randomized rom to complete the level. Something along those lines.
For this specific reason a lot of people just go Ephraim's route when using the randomizer.
Hopefully this helps!