Hi, I was wonder if anyone knows a way to damage a building / unit over time using a location.

all i can think of is a vhp system OR spawn a unit or something maybe like a dt for a second to do some damage

Could just use the "Set HP" action a bunch of times, like set to 95%, later set to 90%, 85%, 80%, ect.

Scarabs are really nice for this purpose. I remember in one map I saw, the maker somehow made it so that the scarab or the explosion was never visible, but it still did damage (I think it had something to do with a bunch of sprites off in an unseen part of the map that made the animations not work).

Set HP might be bad because it might end up healing the unit instead of damaging it (ie, if you attack a building to 50% hp, and the trigger goes off, it would go up to 95%).

No, it doesn't have to do with sprites.

What you do, is put a bunch of reavers constantly firing at a target somewhere on the map. You then move the scarabs via triggers to the location you want damaged and use AI scripts to command the attack and remove the scarab right after. The scarab will cause damage before you see the explosion. Sometimes you might see the scarab itself flash up.

http://www.staredit.net/wiki/Direct_Damage_System_(DDS)