Eridan didn't realize Gamzee had gotten a hold of his arm until he heard the snap. Even then, he didn't look to see how bad the break was nor did he particularly care; he'd had things broken before in his campaigns, particularly when Vriska was involved, and though it'd probably hurt like a motherfucker (to take a tip from Gamzee) later, right now he was too preoccupied with not getting his face smashed in by a psychotic clown to really feel it.

So when Gamzee raised his club, Eridan seized the opportunity to aim his wand again, this time at Gamzee's clubbing hand. With luck, he'd either render that hand useless or just blow the club to smithereens, though his aim wasn't exactly precise right now. And at the same time, he was jamming his foot into whatever part of the clown he could, either to shove him off or just to rack him.

No such thing as fighting dirty when you're fighting a psychotic juggalo.