Okay, dumb suggestion:
Some T1 unit with 2 damage and SKULL for hp (IE, 0hp).
On level up, gains extra damage, so damage curve is 2 ->4->9 or some such. Basically, kind of weaksauce for damageing castle, but VERY good at killing whatever thing it directly in front of it. And... I think it would have good tactical value at various stages of the game, while still remaining kind of weak overall.