Starcraft – As a zerg against a bioball, should you focus fire on the medivacs, or on the offensive units

There is no unit that will magically let you beat a Zerg air mix, even Thors fall to mass Muta with a little micro. Instead let's break down what there is and discuss possibilities:

One of the things you need to ask yourself is: Is he Corruptor heavy or Muta heavy? This question will influence the result. If he's Corruptor heavy you're going to want more of a ground response. If he's Muta heavy, you're going to want more of an air response. I'll break down units for you based on this decision.

Corruptor Heavy

This is usually an indication he's going Broodlords, so its not a bad idea to build some extra Thor/Phoenix/Corruptor

• Marines. Any Terran bio structure will have these. They're more cost effective than Muta and with Stim they'll really tear through them. Highly recommended.
• Ghosts. Zerg air is all bio, and a full Ghost gets 8 shots (360hp) which means that Ghosts eat through Muta. When they're out of energy, you're left with a beefy unit (100hp) with +light damage (and Muta are light). Always a good choice.
• Thor. Without SCV, Thor tend to die to Muta very quickly. What's worse, SCVs get hit by bounce. Unless you have a bunch of these guys backed by 15+ SCVs I wouldn't make them the bulk of my strategy (plus corruption will really kill them).
• Missile Turrets. Surprisingly effective vs Muta, especially if you get the range/armor upgrades.
• Hydralisks. For a high dps unit you can't beat the Hydra. Their low hp means you're going to want to engage on creep where you can abuse your longer range.
• Infestors. AMAZING against Muta, especially with Hydra. Use fungal growth and "lol" at your opponent as he watches his 50+ Muta helplessly die.
• Queens. Mass queens are nasty anti, and the healing means you can win this battle without losing a single one (and not spending any gas either).
• Stalkers. Blink micro is the way to go. If he's big enough to push, don't blink in. Instead blink out Stalkers at low HP, they have a longer range than Muta.
• High Templar. You actually aren't going to get a ton of damage in with these as Muta are very fast, but sometimes you have to work with what you got, and these aren't bad.
• Archon. Too slow, too short range, skip them.

Muta Heavy

Against a Muta heavy force you're going to have more success with air to air as every air to air unit stomps on Muta (and breaks even with Corruptors). You're going to want to back this up with a mineral heavy ground force (Queen/Marine/Stalker).

• Phoenix. The anti Muta; they move while they shoot and have a longer range and faster speed. If he chases you he's dead.
• Voidray. Not the best as the Corruptors will eat them up, but if you can get charged you'll win. To help charge a large force spread them out and do NOT focus fire.
• Carriers. Completely nullify the bounce. Unfortunately you die fast to Corruptors so focus fire them down first.
• Corruptors. This should be obvious: Corruptors beat Muta.
• Muta. Ok, who has the bigger Muta ball. The reality of this battle is, if you can get off 1 fungal growth, you'll win almost instantly.
• Vikings. Longer range means you're going to have to harass with these. Don't get into a straight up battle unless you have backup or superior force.
• Battlecruisers. Not great vs Muta, but better than nothing. Bring some SCVs if you want to win.
• Ravens. Little known fact: most large air forces are decimated by Seeker Missile... with the exception of well played Mutalisks. This is very much an estimation of your opponent. If he hasn't shown heavy micro, this is a good way to go. If he has, it is unlikely Seeker Missile will be useful. It's worth noting that both of the Raven's other abilities work against Zerg Air, so it's still useful to keep a couple of these around.

Remember, if you see Muta/Corruptor you need to think "How can I use what I have?" NOT "What should I build?"

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.