Well, going back to explorer mode I've managed to overcome that combat. Instead of entering the area, I've sent someone there and made him retreat, so that the enemy had to come to me leaving behind the two in the high places and avoiding being flanked. That made the combat easy. Then I defeated the two and only then appeared the giant worm.
I think there should be some difficulty mode between explorer and classic or both should be tweaked, because explorer mode is too easy (but I think that's intended, for those who don't like combat very much) but the classic mode can be a little bit too hard, specially if you don't make the right choices in the beggining (for example, if I had known that a sorcerer, archer and knight/soldier would join me, I would not have made my character an archer; If I knew that source points would only be usable near the end, I would not have had spent money in learning certain skills/spells; etc).
I'll try the same fight in classic mode, now that I have a little bit more experience with this combat.
