I eventually got through using this general strategy:
In Energy Form, focus on rate of fire. Spraying the core with a hundred shots and missing with eighty of them is better than connecting with every one of a carefully-aimed fifteen, especially when the shell around it is moving. When it starts up the lasers, focus on staying inside them and just keep firing, no matter where the shell's opening is; a few are bound to connect, no matter how bad you might be at shooting around corners.
When his homies start to seriously harsh your vibe, switch to Beast Form (via the hotkeys), get them all bunched up in one place, and dive in and out of it with your mouth open until you can see the Jellyfish King again through his cloud of swarming
cnidaria. (This will also heal you a bit, which means you may even have a piece or two of Fish Meat left over when you're done!)
Then again, on my second playthrough my general strategy was to go directly into the Abyss after getting Energy Form just so that I could murder the Jellyfish King and take his stuff. And I thought that strategy made the game considerably
easier. So I may possibly be an utter madman, leaving my advice no use to you or anybody else.
