Lylamwyn studies the approaching wagon and soon realizes that although he might be able to put a fireball down a stationary arrow slit, putting a fireball down an arrowslit that is moving is almost impossible. After that realization, he casts a Fireball above the war wagon in the space between the metal wall and the ceiling.
The fireball does not appear to affect the orc archers (perhaps the wagon has a metal ceiling as well). However, you do hear some orcish screams and the wagon slows down, as if there were other orcs behind the war wagon who were pushing it before being set on fire.
Lenya fires an Eldritch Blast at one of the archers behind an arrow slit, but misses and hits the metal wall (12+ bonuses - 8 arrow slits).
The party heads down the south tunnel to avoid the now slowed war wagon. The war wagon follows behind you out of sight.
After going 140' through a winding tunnel, you enter a large 60' by 60' cave that slopes upward to the south. There are three exits from this cave, west, southwest, and south. Fungus grows all along the East wall and floor.
Several orcs lurk on the far side of the fungus by the southern exit, firing arrows over the fungus at the party. (Craven gets hit for 4 damage, Lylamwyn gets hit for 10 damage).
The party can cross the fungus to attack the orcs. Alternatively, the party can go around the fungus along the west wall to eventually get within melee range of the orcs. Or the party can just engage the orcs with missile fire. Or the party could go around the fungus to one of the other exits (west, southwest).