You don't have to define variables in the variable tab, you can just use any key you want.
However, you should use the Value value type for variable keys, not Game Variable. With Value you define the key directly, with Game Variable you use a text stored in a string variable of the defined key.
Also, the Condition Type in the variable check should just be Variable, since you want to only check a variable. The Conditions condition type is available for checking nested conditions, e.g. A & (B or C), i.e. (B or C) would be a Conditions type needing only One to be valid.
So, for your setup, use Value value type for the variable keys and only a simple Variable condition type checking for it. Also, make sure your Battle component uses the battle start event with the variable change to mark it as a boss fight.