This trick can be used against any enemy unit inside of a tower, but it is particular against shamans, as they usually couldn't get affected by swarms - they only do when they are inside buildings.

 

Start by casting a swarm spell near the building where the enemy shaman is and wait. If the swarm moves to the inside of that building, the shaman will get affected as normal units would, getting scared and running randomly for short seconds, leaving the building or tower. When this happens to your shaman, cast a spell (usually blast or convert), so your shaman stops moving around scared.

 

This trick is really useful in Populous, and is usually used against shaman inside of buildings, and firewarriors inside of towers.

 

You have to know that casting swarm directly on the building you want to affect will never work - you have to cast really close to it, but never directly on it!