No one mentioned reducing the difficulty of harder mobs such as undead mages. I don't know where you got that from. This thread is about the unbalancing or mob ranks. You should be fighting a rank 30 rat on one leve and a rank 30 undead mage on the next, not a rank 30 rat on one leve and a rank 35 undead mage on the next forcing you to reduce its rank to 30 through lowering stars.
Harder leve != higher ranks. Well technically it does, but then you can ask yourself why you aren't doing rank 40 leves at rank 1. The reason you aren't is because leves are designed for a specific rank. So is the star system. The point is balancing this to the point where you know what rank your mobs will be before starting the leve. It doesn't make the mob easier, it makes the leve rank appropriate.