Assuming you have a maxhp or monsterlevel GLOBAL variable for each monster (you don't want an instance variable, because that gets lost when the instance is destroyed). It starts at 0.
Trigger Monster on Death - Increase monsterlevel by one
On monster created - Set hp to 100*(2^monsterlevel)
Or
Trigger Monster on Death - Set maxhp to maxhp*2
On monster created - Set hp to maxhp