Looks awesome, Brad! One thing you might consider is having him use his legs to push himself up when he rears back, it looks like he's pulling his neck back now and he doesn't have a neck? It'll probably work fine as is, but a detail to consider. I really like the details on the legs, by the way.
Also, we might not need side facing directions. There have been many Zelda bosses that only face down, we could have this guy just face up or down, and scuttle sideways like a crab. It looks like scuttling sideways is how his legs are animated anyway.
For the battle flow, I think you mean have the weak spot on his thorax, not abdomen. The abdomen is his middle, the thorax is his butt.
One idea I have that might be cool to work in, is to have carbonites in the room comma and you need to activate them so they roll and hit volcanoraxus to stun him. They would also be capable of cleaning out the small spiders.
There might also be some clusters of bomb flowers growing around the battlefield, and detonating them when the boss is near could also stun him to let you run around and get to his thorax.
My main concern with the battle is that between the small spiders, burning carbon web, and the boss itself, it might get difficult to move to dodge his attacks, allowing the player to be easily overwhelmed. It's probably a good idea to keep the small spiders to a minimum, two or three, to avoid that. And to have a large battlefield.
Anyway, you're doing a great job, Brad : )