I think I managed to go to Demonis, even when there were both Katana (That hero you mentioned), and a stack of unit's. I even got out safely. Also to mention I was 5th Level and didn't use Save/Load. What Level are you? If you're above 12, then you should be fine. Though that depends on whether you have Resurrection spell or not. As for which units to use, I'd suggest some Melee fighters as Archers have low defense and health (Especially Archmages).
Cyclops are okay and maybe if you have Giants somewhere, you can take them as well. As I think that Royal Snakes are one of the best Units there are, and there is a big chance you get lot's of them in Marshal Swamps. Also if you're going to Darion take some Guardsmen. As for the fifth army, take anything that suits you.
Hope this helped a little.
