More seriously, if someone has some oil they could spread it on the ground. Lead the swarm to it and then light it. Repeat until dead.
The swarms tend to be stupid enough to go after whatever food is closest. They also tend to move slower than most characters.
In at least one scenario I played, we ran out of alchemist fire, acid, etc. and the swarm had 2-3 hp left. We finally had someone fast slowly lead it away and then run back to us after it was far away.