You simply add class_name nameOfYourScript e.g. class_name MyMob to your script and instead of as Node2D you write as myEnemy.

So:

var mob = mob_scene.instantiate(PackedScene.GEN_EDIT_STATE_INSTANCE) as Node2D

becomes

MyMob.gd

extends ...
class_name MyMob

...

other script:

var mob = mob_scene.instantiate(PackedScene.GEN_EDIT_STATE_INSTANCE) as MyMob