How to Deal With Banshees in StarCraft TvZ

Spread out all of your troops and buildings. When a Banshee calls in a tactical air strike, it destroys most objects within a certain radius around itself. If all of your troops and buildings are clumped in the same area of the map, a Banshee strike will destroy most of your army and will be devastating. Spreading out your troops will make this less likely to happen.

Build up as many anti-air defenses, such as missile launchers, as you can around your most important troops and buildings. The more missile towers that you place around the perimeter of your army, the less likely the Banshee is to successfully deploy the airstrike. However, it is still possible for the Banshee to have enough time to call in the airstrike before being destroyed.

Deploy units that can specifically counter a Banshee. If you have High Templars, for example, use the Psionic Storm attack to try and take down a Banshee before an air strike can be called. If you've got a group of marines, equip them with Blink so they can detect a cloaked Banshee more easily. The Phoenix units, as well as Corruptors, can also counter the attack of a Banshee.