There are no enemies that are impervious to movement. However, some enemies may be more difficult to move because of their bulk. Thunderwave will be more difficult to use on a dragon than it would be on a kobold fighter if you try to use it. This is represented by the monster’s constitution score. It is usually higher for larger creatures. The constitution is their resistance to being moved against the will of their will. As in real life, it is easier to move a toddler than a man 300 pounds heavier because of the drastic changes in strength and balance.  Although no enemy is immune from movement, even the most powerful monsters can fail to pass a CON check. Before casting the spell, it is important to consider the size of the monster that you are moving.