What's the best way to short Tether?
The correct answer is to do a spread trade between BTC/USDT and BTC/USD since this is where the liquidity is.
The real answer for you is to stay away from this trade. If you need to ask how to do it, you obviously don't appreciate all the complications of such a trade. Remember that this trade will only make money if Tether implodes, and if that happens there will be crazy volatility all around, and you could end up losing money on a seemingly correct trade like shorting USDT/USD.
Is it because the crypto-exchanges tend to go in maintenance mode when a coin is going down?
So even if you consider it inevitable, backing that conviction with your own cash is a great way to get rekt.