Example node hirarchy : Parent/Child/GrandChild
In GrandChild:
var life = 1.0
In Parent:
var grand_child_life = $Child/GrandChild.life
print(grand_child_life)
Output: 1.0
In Parent:
var armor = 2.0
In GrandChild:
var parent_armor = get_node("../..").armor
print(parent_armor)
Output: 2.0
This will always work with that particular hirarchy.