Not sure exactly what you mean, but what you described would look like:
+-global.value(canspawn)= 1
->every 3 seconds system create object sprite ant x,y
-->sprite set value life to sprite.value("life")+ global.value("life")
Thats with global.value("life") set to 0 initially.
When you create the boss set the global to the "updated" value, you can also change that value when you destroy the boss.