I figured out how to damage units who get close to fire using triggers! read to find out:
Trigger 1
Loop, Active, Run Immediately, High Priority
Condition: Always
Damage Units in Area
Radius: Anything you want (though i suggest make it 0 or 1)
Unit: Fire
Player: Any
Percent: 1 (any, actually)
And, you're done! Everytime a unit from the selected player comes near he gets damaged!
Note: This can also work in Acid, Lava Sheet, etc. But i suggest if its lava sheet make the radius higher (i.g. 5, 8) because the unit is wider.