Worldeater already answered to your stop-time question.
To make a unit/units disappear you might move them to a different position. For example, a script that, using a trigger, gets the list of units of side X, then this script, using a tight look, places everything in that list at its current position but with a Z = 4000 + unit current height meters. When a condition is met, the loop exits and places all the units at its current X,Z and a Z = current height - 4000.