All you would need to do to attack a PoS coin is buy up a lot of the shitcoin, transfer them to a lot of different wallets housed is various VPS that all have different IP addresses. You then attack the network, transfer the shitcoin back to an exchange, sell the shitcoin before anyone notices
It is good strategy to attack a shitcoin, but it'll be very expensive, and maybe impossible to buy considerable part of all the bitcoins. And if you have such considerable stake, you wouldn't destroy the value of them by attacking the system.
If you were to buy up a large amount of a PoS coin you would launch an attack, then transfer the coins to an exchange an sell them before the network realizes it has been attacked.