Well I wanted to make a defense but I don't really get what determines how much a certain unit does to another...
Like a ghost with 10 attack will do 2 damage to a ultralisk but another unit with 10 attack will do a different amount.

What's the best/easiest way to balance out enemies and units... do you guys actually just test every single unit or what?

Alright, I'll explain this part to you.

There are 3 Unit Types: Small, Medium, and Large
There are 3ish Attack Types: Concussive, Normal, Explosive, and Ignore Armor (there are more but those are unused)
Concussive does 100% damage to small units (such as ghosts), 75% to medium units (such as Vultures), and 50% to large units (such as Goliaths).
Explosive does 50% damage to small units (such as ghosts), 75% to medium units (such as Vultures), and 100% to large units (such as Goliaths).
Normal does 100% to everything (I think)
Ignore Armor does 100% to everything, but neglects armor points.

Hmm... ok thanks
Is there any editor that tells what unit does what type? I'm using scmdraft 2
i know explosive would be like siege tanks and stuff but other 2 seem similar.

Or is concussive melee attacks, and explosive = gunfire
Just woke up.. cant really think atm

Deathman doesn't know what he's talking about. Concussive is 100% to small, 50% to medium, and 25% to large.

Melee -- Normal damage (yes, it's 100% to all)
Rines -- Normal damage
Firebat -- Concussive
Ghost -- Concussive
Vulture -- Concussive
Dragoon -- Explosive
Tank -- Explosive
Hydra -- Explosive
Goliath -- Normal vs ground, explosive vs air

Do you want to know what air units do? Corsairs are explosive... but I won't bother checking the other ones unless you feel you need it.

I'll add that I recommend not using armor for your units unless your game really requires that your defense ends up with really strong levels at the end. It makes it more tedious to balance. The simpler the better.

There's an ignore armor damage type?
Where?

I'm guessing.. nuke?

Maybe set armor to 0 .No there is not really a ignore armor though.

what about yamato cannon?

Just to tell you guys, there is an ignore armor damage type. Its just not used for any weapon, so its completely useless to mappers .

And, Yamato Gun has explosive damage type. Look at it in DatEdit.

nuke does not ignore armor, but psi storm and irradiate do. Nuke deals (500 or 2/3 max hp whichever is greater)-armor. At center.

Ugh. Jesus.
"No there is not really a ignore armor though."
Wrongplz.
"Its just not used for any weapon, so its completely useless to mappers ."
Semi-wrongplz, depending on what you're counting as weapons.

265/290/295 damage, always. It's one of those 3. Test it in some map if you want to see.

http://www.staredit.net/maplantis/index.php?sid=e92657d2fa;wiki=74

that shows how damage is calculated in SC if thats what you're looking for o.0


edit: o noes... i didnt notice that he quoted it up there >.< *smacks head*

Basically what they've all said about attack typesand unit sizes is right. if you do decide to use armour (though you dont really need it in a def map), note that armour is subtracted after the dmg type penalty

so 100 concussive to an ultralisk (large) which has 5 armour deals:
100*0.25 - 5 = 20 damage

if, after damage type penalties, the damage is less than the armour, it takes 2 hits to deal 1 damage. this still applies if the damage is set to 0, so a 0 damage unit will still deal damage.

Fix'd
Armor is always processed first

That is why using armor MAKES IT EASIER to balance units because the damage type penalties do not weigh as heavy. They do "full damage" to armor then the rest is reduced by the penalty factor of the weapon type.

Also it might be important to note that zealots, firebats, goliath air damage, scout air damage and wall traps, do 2 hits which means armor is subtracted twice from their damage.

We've told you the basic facts that you need to achieve balance. But fine tuning balance cannot be taught, it can only be learned by doing. You need experience.
However I can tell you one guideline: Fine tuned damage and armor values are not as important as keeping in mind which unit does how much damage against which unit(s).
So you should give the players the ability to build and upgrade units of every damage type and send units against him that have varying unit sizes. You can create interesting challenges by starting with some levels of large units and in the end use levels with small units. So the player cannot pump out siege tanks when he wants to survive in the end.
If you want to reduce the amount of work for you, just let the player build units with normal damage. Unfortunately the only ranged unit with normal damage is the marine.

Spells and splash damage can also have strong effects on the balance, especially if you keep damage and hp's of the enemies low, plus you can make bosses when your waves don't already have like 5k hp.

The best way to balance a map is by testing it out over and over anyways.