I take it you aren't using database HP then? You could make a common event which slowly reduces the variable which stores HP, something like:
Loop
Control Variable [Char HP] -1
Wait: 20 Frames
End Loop
If you want, you could put in con. branches etc... It might not be what you want, but if you tell me exactly how you have the HP system set up I might be able to think of a way to do it.