Mmm, use looking_for_help's DDS, after implementing it in the map, there will be a GUI trigger between his triggers, this trigger, remove every single thing but not the if then else things, now, when you want to change the damage, there is a variable called amount, this is the damage, one called target (the target) and one called source (source of damage)
Maybe if you implement the DDS, it will be easier to understand