easy peasy
1 set a zone on the friendly forces you want the carbomb to explode at. name it carbomb
Trigger
ONCE ->unit in zone (unit = XX, zone = carbomb) -> explosion (alt = 0, zone= Carbomb, volume =1000)
then creat another zone called carbomb2 somewhere off to the side....
Once -> moving zone on unit (unit=XX, zone = carbomb2)
Once->unit outside of zone (unit=xx, zone=carbomb) AND unit alive<50% (unit=xx)->explosion (alt=0, zone=carbomb2, volume=1000)
try that