These are the triggers you should need. It also depends on whether you use hyper triggers or not. Ok, here i go
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This trigger will detect whether the unit is in the location, and makes sure the second trigger worksCondition : [player] brings [#] [units] to [location]
Condition : [player] commands 0 [checker]
Action : Set death count of [Timer] to 1 (the timer can be any unit that won't appear in the map)
Action : Create 1 [checker] at [location]
Action : Preserve Trigger
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This trigger will also detect the unit in the location, and start the timer for the consequence, and finishes what the first trigger startedCondition : death count of [timer] is at least 1
Conditon : [player] brings 1 [checker] to [location]
Action: Set death count of [timer 2] to [#]
Action : Remove all [checker] for [player]
Action: Preserve Trigger
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This starts the timerCondition : death count of [timer 2] is at least
2 (so that 1 death count of timer 2 can be the consequence)
Condition : death count of [timer 2] is at MOST 1
Condition : death count of [timer] is exactly 1
Action : subtract one death count of [timer 2]
Action : preserve Trig
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Starts consequence and resets timerCondition : death count of timer 2 is at exactly 1
Action : [consqeuence]
Action : Set death count of [timer 2] to 0
Action : Set death count of [timer] to 0
Action : preserve trigger
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Stops timer when there are no units in the areaCondition : [player] brings 0 [units] to [location]
Action : Set death count of [timer] to 0
Action : preserve trigger
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Well there you have it
Thats how I would do it, but it might be a bit more inefficient then what the other SEN people would suggest. Use mine as a last resort
EDIT : Left my SEN on for a while, and didn't see your post. Sorry about that.
None.