How can I get Siri to shuffle all songs by Phish?
Put all your Phish in a playlist, and give it a name like 'tunesofish'. Then simply say "shuffle playlist tunesofish". Siri will open the playlist and play it shuffled.

Here, I've asked Siri to play my Comedy playlist: 



This works with a smart playlist set to auto-add everything from artist "Phish":