Exchange to sell when the value of the coin drops X%, is that possible?

I'd like to trade in the best and most automated way. I recognize I didn't trade much as I'm keeping my cryptos as a long investment, but I'd like to trade a small amount. But If I want to guarantee my investment, the best way should be to place a sell order in the exchange and sell Ethereum (as an example) when the drop is bigger than 6-10%, and as soon as I sold to place another order to buy when it increase its value over 6%, which it'l mean the dropping time is over.

Is there any exchange where to do it? I've seen Kraken and Bittrex don't allow it.

Is it a bad way of tradding?

Thank you :-)