Amycus probably shouldn't have gotten within arms reach of Rodolphus at that particular moment. He held onto the bottle of liquor so Amycus would have to tug before he released it and with his hands now empty he launched himself off of his perch and caught the other Wizard between his arms as he tacked him onto the ground.
Rodolphus had been half tempted to smash his jar of house centipedes over Amycus' head, especially since some of them were still alive, but there wasn't time to collect a weapon so he just punched him instead.