Time to Go

After foiling the onslaught of the earth elementals and accompanying forces, the weakened structures of the fortress walls (if still intact) collapse, causing 3d6 bludgeoning damage to anyone still in the building. The characters are able to get into the dungeon before the collapse, but still take 1d4 bludgeoning damage from debris falling throughout the dungeon.
As the dust settles, word comes that the svirfneblin army has arrived. A hasty conference will suggest that the options are closing down, and that it is probably best to plan an escape before the kobolds bring in any more reinforcements. The svirfneblin are willing to aid in your escape, but have no interest in a protracted engagement this near to the surface. They will make a feint to give you an escape option, then fall back into their own tunnels, drawing the enemy in or collapsing the tunnels to seal them out as they think will work best.
A careful search of the area near the stairs up into the fortress building will show that it is completely buried above. The only apparent options are to go into the svirfneblin tunnels, or make a break through the lower entrance. Encourage the party to do a careful search of the tunnels for another way out. With the collapse of the buildings above, part of the ceiling in one of the rooms nearest the stairs (next to the kobold tunnels) has fallen in and reveals light coming through.
Checking this out is promising. With the buildings all trampled, and access to the tunnels from above cut off, the kobolds have left the hilltop to gather for an assault at the lower entrance. There is a small group of 6 kobolds and two bugbears left at the ruins to keep an eye out. Knowing they have sealed off the entrance, they are not paying very close attention to anything on the hilltop.
Night is falling quickly, and everyone is poised for an almost certain night assault. Studying the movements of the few opponents above will show them huddled near a small fire about 50 feet from the edge of the ruined walls. They appear to have found a wild boar that they are roasting over the fire, while they divide the spoils they have gathered from the battlefield.
The gnomes will aid you in slipping out into the night through the partially collapsed ceiling. Have the characters make Stealth checks with advantage against the monsters' passive perception scores. Anyone who makes a successful check is able to slip out and take cover amid the debris. If someone fails, the bugbears head over to check out the noise. Let them roll a perception check against their stealth checks for them to remain hidden. If they notice nothing, they will return to the fire. Otherwise, they will approach whoever they notice.
If the characters are quick, they can overpower the small band, and slip away. Once they have taken the hilltop, they can survey the valley below. At some signal, the bulk of the forces below move around the hill toward the lower entrance. The svirfneblin pour out of the entrance to engage the kobold army, and draw all of their attention to that area. This opens a path down the hill and across the valley. The players need to make three or four perception checks to notice roving guard units, followed by stealth checks to avoid detection. They have the option of fighting, but doing so will create noise and draw attention.
If they engage two or more of the groups, it will trigger an encounter with a tougher band, led by a powerful sorceress.