Galactus was only killed when the Surfer turned his own planet devouring machine against him. So basically, he was only killed by a machine he created because no one else in the Marvel Universe who has any reason to want him dead can actually kill him (and doing so is a bad idea any way since his existence is an integral part of the universe).

Onslaught has nowhere near an unlimited amount of strength, and his telepathy also can't be limitless. He lost in physical combat to the Hulk because the Hulk was stronger, and he lacked the telepathic power required to turn the human race into a collective consciousness as he had planned, needing X-Man's power to be able to pull that off. He was certainly the most powerful telepath on Earth, but that wouldn't mean much to Galactus; a being who can teleport galaxies, create sentient life, and manipulate souls (among other things) with ease.