Let’s come up with a monster.

Neutral monster - Werewolf
Can transform into any human unit during battle (the unit level depends on the number: 5 - archer, 10 - crossbowman, 20 - swordsman, 40 - pikeman, etc.). Weak against mages and level 7 units for the following reason:
Mages revert it to its original form (with a hit), and level 7 units remove the form and deal double damage.