Beast form with pistol shrimp shots is a very formidable weapon - the only disadvantage is how much effort goes into eating the creature. The following is a quote from Glamador on the Dual Form:
It's not particularly useful against basic enemies of aquaria. But in the body, and against mini-bosses, and really anything with above average health, the Dual Form really shines. Li's shot is SUPER powerful and even against mini-bosses the scream HURTS. That's what's so great about it. It's pure power. Of course you shouldn't use it against FISH but against the super-nautilus's in the body? There's nothing better.