ha, success. Just for clarification, im guessing when u say timed life, u mean something like the MULE yes?
The timed life effect is done using the data manager. By creating a buff which destroys your unit after a set time (refer to the MULE's timed life buff for all the info u need), u can set it to apply when that specific unit is created.
Using triggers, u can apply that buff to occur when a certain event has occured, such as a unit entering a lava field, and then removing the buff when the unit is outside the lava field. This cud be done using regions for an example.
if u have some basic knowledge of some triggers, im sure you could achieve this effect
if not you will need someone more experienced than me or read up on some tutorials which use similar trigger events.
I hope this was what u were asking for (because ive never played WC3 so i have no idea what you were referring to) and good luck in making this effect. (remember to take a look at how the mule's timed life works)