Monkey's paw it. Give them a poison first session that makes them permanently lose control of their shifting.

Make their class random every dawn. For for chaotic madness, make their class randomly shift every 6 seconds at the beginning of their turn. Or make it change to the least useful class for the particular situation. Or maybe the player doesn't even know which class their charcter is at any one time and must decide what to try before they know if they're even the right class to try that. ("You gesticulate and make a constipated face and feel at one with the forces of... rolls dice... smashy smashy. You've forgotten what a 'magic missile' is.")