QUOTE(LegacyWeapon)
Make it,
Condition: Death score for current player is exactly 0.
Actions:
Create blah blah blah (or any another action)
Set deaths of unused unit to set to X* for current player
and make another trigger,
Condition: Current player's death score of unused unit is at least 1.
Actions:
Set deaths of unused unit to subtract 1 for current player.
X should be 100*(number of seconds)
Only if you use hypertriggers. Otherwise it will be slow as hell.
Credit to LegacyWeapon for this information.
Adapt this to your need. In addition, you may want to make the death counters cycle constantly (which is shown in Legacy's first trigger).
However... for the first trigger on LegacyWeapons list, I recommend you do 'suffers 1 death of unused unit' instead of 'suffer 0 of unused unit' because the death of every unit in every game always start with 0 deaths.
With hypers, 12 deaths is deducted per second.
Without hypers, 1.5 deaths is dedected per second approx.
Always test to maker sure they run at the speed you want.