Starcraft – How do units choose which enethe to target in SC2

Starcraft 1 has a different psionic storm than Starcraft 2, so I'll only be answering Starcraft 2

It is 80 damage spread out over 4 seconds. Psionic storm is true damage and is not reduced by abilities or armor. Any unit caught under the storm (friendly or otherwise) will take 80 damage if it remains for the full duration. Its worth noting that Zerg units at full health will regenerate 1 HP when first hit. Storms do not stack. Any unit under a storm gains the storm effect. It is this effect which conveys the damage. You can only have 1 storm effect at a time on a given unit.

Depending on your army composition running through storms is often recommended you run from storms. Because there is a slight animation delay (1.5 seconds) you can negate a large portion of the damage if you act quickly.

Storms only hurt units. Units inside bunkers or carriers or other transport are unaffected.

Storms have a range of 8.25 to 9.75 as they have a radius of 1.5. Smart cast means the closest templar will energy will cast the storm.

Focus fire is critical for sniping injured units and for taking down important or high DPS units like void rays, ghosts, brood lords, immortals etc...

But beyond that, it actually worse to focus fire units. Using this awesome map, I have actually tested micro battles with and without focus fire. I have found that 9/10 times any group of units I micro to focus fire just flat out lose to non-micro'd units.

There are two problems with naive use of focus fire. 1) Targeting one unit will most likely cause you to over-kill the unit, which is a waste when you could be shooting at another target. and 2) If your unit is not in perfect range of the target, the unit will bumble around until they are, which means instead of attacking, they are bumbling around.