A Crushing Blow

After a few hours, another assault is made. This time, the 30 kobolds are joined by 6 bugbears, 6 hobgoblins, and 2 earth elementals. Around midday, a shout goes up from the forces gathered below, followed by a ripple of laughter and taunts as the troops make their way up toward the fortress.
The kobolds rush forward, with the bugbears and hobgoblins following in a more ordered march. The earth elementals lurch forward in a surreal fashion, almost seeming to glide over the earth and through all rocks and natural obstacles in their path.
As they near the fortress, the kobolds lose their enthusiasm, especially coming in view of the scattered carcasses of their kinsmen. They drop back, allowing the others to move past, falling in step behind the elementals. The hobgoblins and bugbears are less skittish, but they also seem to slow as they approach. The elementals continue to glide forward.
The elementals head directly for the most intact buildings, working to bring them down to rubble. The kobolds and others will fan out behind them to attack anyone that tries to stop or interact with the elementals.
If the party was able to recover one of the ballistas, they can use it against this new assault. Otherwise, they will have to find another way to deter the assault.
Meanwhile, the svirfneblin are facing an attack from the kobolds in the tunnels below. This battle can be managed as the DM sees fit. I feel like the best option is use the conflict as a means to keep the gnomes busy and block access to the players from the tunnels. Given the release this week of rules for mass combat, this could also be a way to try those out.
It is probably the time to bring this whole scenario to a conclusion. Tomorrow. As a setup to that, it is probably best to allow the elementals to destroy the buildings above, and have the main svirfneblin force arrive at the conclusion of these battles.
Tomorrow, we'll figure out a way to engage the main force and allow the players an opportunity to slip away. I'm blank at the moment. They could escape through the svirfneblins' access point to the Underdark, but I don't like that. It feels too much like an ex machina solution.