A little help?

Our current campaign with DM S is coming to a close, and while I'm sad to see it go, I am excited for a campaign that actually starts at level 1 and doesn't skip 2-5 levels at a time. ><

Unfortunately as a close to the current story, we have a bit of a puzzle:

We are in a city full of religious zealots, tasked with assassinating as many people in power as we can. We're level 18 and our party consists of the following: Barbarian, Wizard (Illusion school), Cleric (Trickery Domain), Fighter (Champion), and Druid (Circle of the Land). The druid and barbarian have been captured and the high priestess of this place is guarded by a solar.

We've attempted several plans, managed to kill several innocents and lost more than half of the soldiers sent to help us. We even managed to use the Gate spell to summon Geryon in an attempt to fight the solar, but he was ultimately sent back to his domain.

Any ideas? We'd definitely appreciate them!