I want to make a trigger where when a unit reach 50% of there hp, they will say a message on the map. Can that be done.?
Nope, only EUD condtions can do that, and im pretty sure the new patch scrambled them, there is no condtion that can detect health
yes, this will work when EUD conditions work again(hopefully). however, it isn't very practical. you must known the unit's creation value(wrong term, but it'll suffice), which is essentailly the units place in a line of all created units.
you can't use location or generic unit types(such as marine, it has to be 'marine that was the 3rd unit created' - ps not third marine created, third unit period).
if you still need help or elaboration, just ask.
Virtual Hp -- end of story. Thats pretty easy to do so if you need help get in touch with Zeratul coz i know hes a good mapper or you can get in touch with me either one ^_^.