The mutalisk's attack strikes three targets in succession, dealing 9 damage to the first, 3 to the second, and 1 to the last. Unlike many other air units, the mutalisk is small with light-type armor, making it more resistant to explosive damage, a common damage type found on anti-air units like dragoons and goliath. The mutalisk deals normal damage and is only affected by the armor of the target. Compared to the wraith and scout, the mutalisk's attack is weaker and more easily affected by armor.
Large groups of mutalisks tend to clump together making them ideal targets for attacks or abilities dealing area of effect or splash damage. Mutalisks are fast, but accelerate slowly making it harder to escape; this improves the effectiveness of the high templar's psionic storm against mutalisks. archons are likewise a good counter against mutalisks due to the splash damage of their psionic shockwave. On the other hand, stacking mutalisks does make them easier to control, and can make it harder for single target attacks to deal with them (e.g. marines). One can trick mutalisks into stacking by including an overlord in the control group.
Corsairs and valkyries are also effective against mutalisks due to their splash damage attacks.