You have to be quick. When Karkinos gets up, eggs hatch. Not a bug, as intended. You have to be quick, grab the egg, roll it a little bit forward, target the up-side down Karkinos, and middle-mouse-button fire that egg right on him. Then attack him. Very tricky, needs terribly precise timing, very big annoyance. Doable, though.
If that is the case it was not very well planned.
If the egg doesn't hit him, it was too far away from him when you fired it, your targeting was a little off, a hatched crab from before might have hit you while you tried to hurl the stone, etc. You have many hindrances during that fight. When he charges towards you, though, try to run to the exact opposite end of the screen, because there is where the stones spawn. Beginning to run to there just the moment he begins charging, is key in preserving you enough time to do the positioning, targeting and stone hurling later.
Also, remember to pick up his maul loot drop. It is a great item that will get you far, it hits heavily enemies.