The thing about a company is that its share price doesn't have to be related to the health of the company. Lots of companies' share prices dropped 50% in 2008, but that didn't mean that the firms were struggling, it meant that people were afraid of the firm struggling, or even merely afraid that they wouldn't get their money out so they sold at a loss to get something. Imagine if Coca Cola (KO) dropped 50% today. Is KO struggling? Not at all. You have cause and effect backwards. The firm deemed to be struggling tends to have its share price decrease. And those that can tell the difference between a company that appears to be struggling and one that actually is struggling make money.

Unlike most companies, though, which provide tangible goods and services for which people pay and has factories and stores and real estate you could sell, Bitcoin is just a list of numbers floating around.

Some exchanges allow this.

The safest way is probably to sell futures on CBOE or CME (those are the exchanges, but you'd do the transactions through your broker). I say safest because you don't have to worry about counterparty risk from a sketchy offshore Bitcoin exchange. The futures are cash settled based on the price of bitcoin, much like SPX or similar equity indexes.

Obviously there's a ton of risk involved, there's no reason it can't double for a dumb reason and you get caught owing a fortune.