In the end the market found floor at 10 cents until all margin calls and STOPS were cleared. The only way one would have caught a 1 second, 2500% crash is if they had standing orders at lower prices, including 10 cents. The GDAX / all other exchanges trading Ether were already stalled due to frantic ICO funding requirements. Example: Civic ICO wasn't accepting Ethers, I believe. In the end markets were thin, and those with STOPS and margin traders got wiped out in 1 second before price reverted to normalcy zone as there wasn't really any true selling pressure other than the 1 big order in millions, which shouldn't be a big deal for a multi-billion dollar market cap ETH. I would think that this big order of millions got extremely poor fills and perhaps didn't settle for "millions".

Short answer is you would have had to have a standing order at 10 cents to have caught the flash crash. Which for anyone to predict that ETH will go from $300+ to 10 cents in 1 second or even in a day is next to impossible. Hope that helps!