Hello, I also have a solution for your problem and your trigger should look a little something like this. I have also tested this out myself and it works!
Your Trigger
Events
Time - Every 1.00 seconds of game time
Conditions
Actions
Set
ManaLoss = (Units in (Playable map area))
Unit Group - Pick every unit in ManaLoss and do (Actions)
Loop - Actions
If (All Conditions are True) then do (Then Actions) else do (Else Actions)
If - Conditions
(Mana of (Picked unit)) Equal to 0.00
Then - Actions
Unit - Set life of (Picked unit) to ((Life of (Picked unit)) - 1.00)
Else - Actions
Do nothing
ManaLoss Is a
Unit Group variable
Same Example just in a picture
If you have any questions please let me know! I hope this has been helpful