Unfortunately, shooting fire at an already flaming zombie is merely an exercise in futility. To give Eggman credit, he actually DID melt away most of Knuckles' remaining brains into slag with his plasma gun, but a mindless zombie generally doesn't care about things like that, so Z Knuckles continued on, with one flaming fist punching the plasma gun into a folded-accordion shape.
Sonic, meanwhile, was still being attatcked by Chaos, so he wasn't able to either mourn for Knuckles or jeer at Eggman at that moment. Instead, he was whirling up a super-fast tornado around Chaos, turning the water spirit into a typhoon and drastically raising the humidity level. The swiftly flying water also shorted out the Egg Worm's plasma cannon and doused Z Knuckles' flames, which didn't make ANYONE present the least bit happy except for Sonic.
