I think we shouldn't overestimate the internal damage done by Killer. The advantage of Sonic Blades over Gamma Knife is, that they cover a larger range. But even though they went through this whole body, his organs seem to be damaged only superficially like with impact dials. And his eyes were blurred out by both Sonic blades and Gamma Knife.
Doffy on the other hand also suffered internal damage from Red Hawk; the fire went through his body and came out of his back, burning the insides. But Doffy stood up quickly.
Also, even if Killer can kill Doffy with his Sonic Blades, doesn't mean he will lay a hand on Doffy. Doffy is pretty agile and versatile, controls the environment, flies and can immobilize you with parasite. That's why Law had to play dead in order to land Gamma Knife.
By that logic each Scabbard is stronger than Doffy and we saw how Doffy fodderized Kinemon. Kinemon was in awe seeing Doffy, lol.