Large raid on enemies hideout

I need advice on how to run a rather large encounter that is in between a heist and a raid.

The local faction headed by the BBEG has been disappearing people from the city the PCs are in. Their hideout is in some mountains just outside of the city. Other groups (the local druid grove, members of an opposing faction, a civil protection guard made of volunteers) have all scouted the place and realised it's too heavily guarded to be attacked.

I am going to have several swarms of enemies (mostly swarms of shadows and other guards using the swarm mechanics) patrolling the surroundings and the entrance to the cave where the hideout is, plus some traps. If they find a way in, the rest is a dungeon crawler.

How would you run this? Do you have any ideas or inspiration?