I'm not sure exactly what would it take. I think all Steem seed nodes would have to be DDOS at least all of them used by witnesses. The seeds provide the state of the blockchain (latest blocks) to the witnesses. I think DDOS the witnesses would be more tricky though I'm not sure. I think RPC nodes would have to be DDOS too.
I'm not an expert on these kind of things.
A list of some of the public seed nodes.