[Taqwus]

Consider a unit that represented an archer or crossbowman shooting through an oversize shield, for instance -- it could have poor mobility, poor defense, but very high parry reflecting resistance to missile fire. Not too freaky.

Freakier would be giving that unit a #secondshape with no mobility or attacks, mindlessness, and somewhat reduced parry to represent an abandoned and damaged shield (dead archer). It might still be useful for a size-2 unit to enter that square, but I'd have to check the missile targeting rules.

More magically, fire-spitting machines aren't too freaky, and there's already the fire + water = acid theme, so short-range acid-lobbing (perhaps big cauldrons with a one-shot short-range acid breath; likewise for nature/poison; minimal hp + second-shape into a low-morale crew running away?) might be thematic. Water might lead to an Earth Meld effect. Allowing a construction to cast Rain of Stones might be a bit harsh, but hmm...