Hmm, my bad, I thought you can generalize the Unit's life becomes = x event in GUI but it looks like it's a specific unit event.
Well, you should stick with the negative mana regeneration at all costs, that makes setting the unit's mana in the trigger unnecessary. So you should only use that trigger to check if the mana of a footman is 0 and if it is give it a negative hp regen ability. Oh, and I wouldn't check it every 0.25 sec. every 1 or 2 or 3 sec would be perfect and it would be less strain on the computer's resources.