When Rock Crab puts his arms down to fire energy blasts, dodge the blasts and fire away at his eyes to daze him. When dazed, he'll drop his arms - whale (pun intended) away on his armpits. When both claws are gone, quickly swim over Rock Crab to get him to leap - be underneath him during his leap, and fire shots upwards to finish him off.
Sorry for the long kill, here - I ran into a rather obnoxious problem where I could not stop targeting his left (our right) armpit, even after the claw was destroyed. With some clever maneuvering and positioning, I was able to (eventually) take out the other claw as well.
(After the kill, I thought about changing out of Energy Form, then back into it - it may have reset the targeting...but oh well; too late now xD)
Steam Achievement: Rock Crab Rocked - Defeat the Rock Crab.
Disclaimer: I do not own, or claim to own, any part of the displayed content - the art, music, etc...was all created by, and is owned/copyrighted by, the independent developers over at Bit Blot - the creators of Aquaria. I have no affiliation/association, or claim to have any affiliation/association with BitBlot. I am nothing more than a customer who purchased Aquaria on Steam, played it, loved it, enjoyed it, and wanted to post a play-though for Aquaria.