If you throw something like that down, anyone unlucky enough to be hit will get hurt.
Literal
That-kind-of thing [object-を] if-throw-down, unluckily hit person [topic-は], injury do.
投げ下ろす is a compound verb: 投げる (throw) + 下ろす (bring down). The ~ば conditional sets up a natural consequence. 運悪く is the adverbial form of 運が悪い (unlucky). ケガをする uses the katakana spelling of 怪我, common in informal writing.