Is there a way to subtract health? I know the way to SET health, but that does me no good.
No, the only way to do that is have a variable representing health. It's a tricky and often debated topic. There is no simple solution. Sorry.

Doom Dragoon once posted a map on blizzforums that creates an irridated unit next to yours.
So if your unit is organic then it would slowly subtract health. Thats about all i got. :/
You could move the unit to another area of the map where it will be attacked then move it back. Have maybe a unit that does 1 dmg every 1 second or something, then move the unit there for 3 seconds to subtract 3 damage.
I did something like that for a healer deal. Made a set of triggers that would heal your unit by 10hp each time you went to the beacon.
You have to first kill the unit of detecting how long it took to kill it and how much damage that relates to. Then respawn it according to the health it had plus X. Oh well lol
Here's my map, subtracts hp randomly based when your lurker dies... It's worth a look I guess... Keep in mind that it uses custom points for hp, which is A LOT more flexible than regular battle systems...
well i did it once but now i forgot how to sry :/

To heal, you could try spawning a medic for a friendly computer for a short time.. or a unit for a hostile computer to damage it.
Well... darn. The impression I got was that subtracting health is either impossible or really really hard to do. Guess I will have to do with out. As for using custom score for health, I'm doing that for a repair trigger, the more your hurt, the more you pay.
In case you wanted to know, I'm making a new map, tenitivly called Mech Arena. You'll see it when it comes out, hopefully in the next week or so.